if i’m correct that’s the same engine as in my river 600, but with 44hp. Parts are hard to find, but there have some in stock. http://www.moromotoricambi.it . A bit pricey, but cheap in comparison to dumping the bike. Starter motor breaking teeth on the motor side? That sounds strange to me, please explain it a bit more.
I haven’t heard of any mod from the factory nor kick starter conversion, but sounds very interesting, if you get info and don’t mind sharing, please contact me or post here.