Welcome Pete!
Trying to find an exact wiring diagram is not going to happen.
Since it is no longer factory, it will be a little different, but not difficult if you have a basic understanding of wiring principles.
Some things that will change, being a 12v with an alternator, it will now be negative ground.
The starter will remain the same. Just be cautious not to crank too long, and be careful engaging the starter, being sure it has stopped spinning, and the engine has completely stopped before engaging.
If it still has the 6v coil, it will need a 1.5 ohm resistor ahead of the coil. If it has a true 12v coil, no resistor will be needed.
Everything related to the generator and regulator can be eliminated.
The most common and economical and practical alternator is called a "10SI" Delco. It comes in a standard 3 wire model, or a modified "One Wire" configuration.
A one wire is just what it says, only one #10 wire that wires directly to the + terminal of the battery, or amp gauge, if equipped.
The 3 wire is a little more involved, but not difficult. See link.
The start lever, that needs to be incorporated back into the start circuit. The purpose is to provide the neutral safety feature. Very dangerous to not have that operating. You know better, and will always be careful, but will the next person driving it? Grand kids playing on it?
Originally there was a heavy duty set of contacts that directly wired to the battery and starter. They were troublesome. The contacts can be bought aftermarket. Another solution is to add a solenoid, use the contacts to energize the solenoid. That gets the load off the contacts, just more reliable, same end result.
On the left, there is a list of topics. Click on "Articles", scroll down through the topics. There are several on 12v conversions, other wiring topics. Lots of detailed info there.
Don't be afraid to ask, you can do this, it's not difficult. All the wiring can be #14 except the charge wire from the large post on the alternator, it needs to be #10. Use stranded automotive wire, not solid or building wire. You can get as detailed as you desire, soldered and shrink wrapped connections, split plastic wire loom, tie everything up and secure makes for a trouble free and professional appearance.
Let us know...
PS: Depending on the current condition, check the gear cases for water contamination, very common to have water in them if stored outside. Good chance the gas tank is rusty and contaminated, it may need to come off and be rinsed out with E85 or replaced if flaking rust or leaking. Try to get it running, drive it around, make a list of everything you find wrong. Best to have a game plan, fix what is needed the first time in instead of finding more problems and having to back track.
A shop manual will be a very essential investment. It will pay for itself in mistakes not made!
10SI Wiring