There are more parts available than you may realize; a lot are common with the 650. At the very least, stuff can be adapted.
Case in point, the electric starter system. It's all quite common with the 650. It is a simple system, too. There is the starter motor, a pair of reduction gears, and a big-ass ring gear with a sprag clutch.
The reduction gears are also an anti-kickback device to protect the start motor. They are designed to slip under a heavy load.
Going through the starter motor driveline, here are the things that could be wrong.
The starter motor is making some kind of noise, but is really not spinning.
The spur gear that is on the starter motor shaft is completely stripped.
The reduction gears' anti-kickback feature is slipping.
The sprag clutch is shot.
There are stripped gears somewhere in the drivetrain.
You don't mention any horrible and depressing noises while engaging the starter, so stripped gears would seem unlikely.
The reduction gears are pretty robust and it would seem like there would have to be something wrong with the engine in order for those things to screw up, but it's worth checking. @pdwestman
may have some experience with high-mileage reduction gears. In fact, I hope he'll give his opinion on what the issue could be.
The sprag clutch is easy to check. It should easily spin in a clockwise direction but should lock up solid in the counter-clockwise direction.
Unfortunately, you have to take it apart to find out what is and isn't working right. The parts are not terribly expensive.
Have you started it with the kick-starter or by bump starting it and does it run normally?