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My 2072 will not turn over. Replaced started solenoid, hoping it was a quick fix. But it just clicks when key is turned. If jumper wire is used directly to starter, the starter will engage. I cleaned all connections, still nothing. Is there another relay? This one did not have to press pedal safety to start, nor be seated, so no switches there to go bad...

Dan - Anyone giving you an answer is guessing. Electrical troubleshooting is a combination of understanding how it is wired and using a multimeter to work through the schematic. Generally the culprit is some form of a bad connection. Any connections that are suspect need to be cleaned then have dielectric grease applied to prevent future corrosion. The corrosion that is stopping the flow of electricity is sometimes not easily seen. I've had battery terminals that seem ok, but cleaning has made a difference.

Since you're asking for guesses, check the ignition switch. They are notorious for developing high resistance inside and also the backside connections can get corroded or heat affected by the bad switch.

It can be difficult for someone less experienced with electrical troubleshooting because the connections can act differently with the high amperage draw of the starter vs. the small battery in the multimeter.

If you get to your wits end, and you are willing to waste some money on a guess, replace the ignition switch. Make sure the connector is good and clean, and that all the terminals are tight. Apply dielectric grease to all those connections.

Good luck, it can be frustrating.

Tom is hitting everything on the money. they can be tricky to diagnose. And to top it off they can also trick you into thinking you have a good connection.
Testing things with a Volt meter can sometimes show a good 12 volts at a test location- most people are like ok i got 12 volts there im good right? wrong!
Alot of times when a load is applied the voltage will drop to nothing which means u have an issue.

So u jumped the starter and it turns over. Likely your ground to engine is good. battery is good, starter is good.

Next thing i would try is to jump the small terminal on solonoid to 12 volts. IF engine turns over then likely your Heavy cables and terminations are in good shape. and your problem is in the "start" circuit.
IF you determine its in the start circuit, download the wire daigram and go step by step. start circuit goes from B+ through switch, safety switches pto switch and to solonoid.

link to diagram

2072 Wiring Diagram

As alluded to by the good folks above these tractors have a simple electronic system BUT have complex issues because of it. No start with faint or no solenoid click will 75% of the time be a bad ignition switch. Why, because they were CHEAP! Instead of using a brass contact for the switch they used brass plated steel. Over the years the vibration causes the connectors to fret and the plating erodes and corrodes and that increases the resistance which just accelerates the problem even more as it creates heat in the connection. Out of all the cyclops tracts I've owned I've had 1 that didn't have a melted mess on the back of the switch. When you look at the system it becomes easy to see why. The switch controls all of the charging and load current for the tractor. The only load it doesn't have is the starter itself.
My SOP with them is simple, Buy the tractor, remove the switch, replace with new and add new connectors for the wiring, then slobber them with grease, install and coat the entire thing with grease, then wrap that with a chunk of bike inner tube slid over the wires. It usually stretches a bit around the switch, but a zip tie there and another down on the harness seems to do the trick. Cures 90% of the no start or poor lighting issues.
These days I go a bit farther and install a relay assembly with a rectifier and better connections along with a 4 fuse panel to make things better. One fuse is the master for the charging system, another for the lights, one for the clutch and the last for accessories unique to each tractor.
The way they should have been from the factory.
Testing can be a pain but the first thing is to jump it like you did and verify the battery cables and ground side are OK, as it turns over OK that way it points to the switch or one of the interlock switches. Look at those if they were bypassed as some bypass methods work better than others. I've seen wire nuts, wire stuck into the connector and taped in place or someone taped a switch into position. A common "solution" under the seat was to remove the bolts that hold the spring loaded switch bar, place two washers under them and reinstall the bolts to lock the switches in position. It works but the switches can corrode internally.
Next test is to pull the solenoid trigger switch off, connect a heavy draw test light across it (got a working headlight handy?) connect it between the trigger and ground and hit the key. Nice bright light? Check the starter solenoid for problems. Dim or no light head for the PTO and ignition switch.
PTO switch in OFF position puts current through the C and D terminals then to the brake switch and around to the starter solenoid.
Power comes into the ignition switch on the B (battery terminal) It's usually the one that causes problems as well because of the current flowing through it for everything. If you get a new switch be sure to get the correct one, they used basically two switches in the cyclops units, one for the engines that used battery ignition and one for the magneto style ignition. The difference is the M terminal you need, when the key is turned off it goes to ground to stop spark, on the other version has an I terminal and feeds power to the ignition system.