I worked as a mechanic for a Bolens deal when those things were new - or almost new. 600, 800, 850, and 1050 tractors stick most in my mind. I though the 1050s mostly had 10 horse cast-iron Tecumehs - but maybe you've got the Wisconsin TRA-10D model?
Those tractors were very rugged - but still fell apart quite a bit. Also, the worm-gear drive in the rear-axle can be a problem in many ways. First - I suspect you cannot buy a new bronze drive gear anymore? Also, they won't roll unless you pull the disconnect pin.
In regard to changing engines - yeah you can put just about any horizontal engine on it with a 1" shaft. OEM, Bolens used Kohler, Briggs & Stratton, Wisconsin, and Tecumseh - and we had the most trouble with the cast-iron flathead, electronic igntion Tecumseh engines.