Kohler have their training course on line
Read the manuals a few times then do the test
You end up with a print yourself diploma,
Looks good on the wall.
Apart from that & what has already been mentioned, get a John Deer technical manual.
Each section has a theory section at the beginning to tell you how the mower works then there is a detailed test section telling how to test each part and more important what order to test them in.
All tractor style mowers are basically the same and the 100 series JD comes in both the cheap manual PTO & dearer electric PTO so that will have you covered for all tractor style mowers.
ZTR's are a bit different .
Mechanically they are all the same but the electrics are quite varied, so no one manual will cover most , let alone all of them.
An engine is an engine is an engine if you can fix one you can fix all of them, they all work the same .Only 2 types really OHV & SV and now days not many SV's out there.
What is going to give you grief are the small hand helds.
They are very very fussy.
The first few sections of the Husqvarna service school are on U tube on Joe Pace's web channel.
After that there are a lot of U-tube channels that provide really good information and good work shop practice.
Donny Boy & Taryl fixes all are both worth watching from start to finish, several hundred of them.
After that I have problems with either the methodology or presentation of the bulk of the remainder as nearly all of them assume that you know what the actual problem is & make lottle effort to explain how they worked out what the problem was.
Diagnosis is the hard bit
Repairs are a cake walk.