The .61FS is part of the generation of engines prior to the surpass series that is now discontinued. The engine you have is easy to run and durable but a little less powerful than the surpass engines of similar size. As old as this engine is now, condition is going to determine its value more than anything else. If it just needs cam and crank bearings, and a good cleaning? Maybe $60-75. I have seen brand new in box examples go for $125-135 or so.