A longer governor spring would reduce the speed, Peter - it sounds as if a previous tenant had shortened it, or it was the wrong spring.
I think "difficult refurbishment" is something many people experience with side valve Tecumseh engines, mostly because of the carburetors. On the other hand they are mechanically robust and very durable. Back in the 1950s, when people expected such things to last a couple of generations and didn't really mind having nasty experiences with the carburetor, it was probably the right engine for its time.
