John deere 108

Anybody have any good ideas where to find an original engine for a model 108? I have been looking just a little bit and cant seem to come up with anything close to a model 191707 briggs. I know T-Mo(Terry) has said it may be a hard task to find an engine. I bought this tractor in non running condition for $25, but it has a 12.5 hp briggs in it not running, which I would like to bring this one back to original. I know these tractors arent collectable now, but if I keep it long enough some day it may be collectable. I know I havent seen a whole lot of the model 108 so I like the ones I dont see very often. Any info would be appreciated.

Thanks a lot,
Blake