I had an old control line plane given to me and it had a fox 35 40th anniversary engine on it. When I got it I couldn't get the propeller to turn at all. I took the head and back plate off along with the propeller and soaked it in kerosene for a couple of days. It turns over now but it seems a little stiff. However, I really have no reference to what stiff is so I don't know if it's still gummed up or if it just has good compression. How can I tell if this engine is worth buying fuel for and spending the time to get it going?