Can not find an operator manual using your 1991 model number but did find a manual with the model listed at this sitehttps://lookup3.toro.com/request/request.cfm
Use R1-12K804 for a model number and 2012345 for a serial number. This is a 1992 model but lists the 1991 on the manual. After all that it has no wiring diagram in it.
You can download a detailed wiring diagram for the 1991 North American models from the Files ofhttp://groups.yahoo.com/group/wheelhors ... anuals3_1/
Membership is required. The file is named
Tractor 1991 312-8 31-12K802 Wiring Detailed
These pages are taken from the 523 page Demystification Guide we can download from Toro. They are easy to understand as each circuit has it's own diagram. I can not find the engine spec number for your tractor to verify these are correct for you but quite confident they are the same.
A word of caution. Your ignition system is self powered so do not accidentally allow any battery voltage into the ignition circuit. It will cook the trigger and coil - an expensive repair. 1/2 of the pto switch and the seat switch have the ignition wire in them. To stop the engine that wire is simply grounded to the chassis. Open circuit and it will run. Be careful when testing.
Keep us posted on your progress.