There were a lot of different variations on the wenmac, but I have never had one that was not a decent runner. Some of them really nice. It came as quite a surprise, as I would have ASSUMED them to be all junk, considering their price. Actually very nice running engines.
If you have one that has a built-in glow head, as opposed to a head with a glow plug, try a cox one. I think they fit. And the more nitro, the better.
Don't bother with fifteen percent or anything. Cox fuel, or even better, cox racing fuel.
They have very little collector value, so run it and have fun with it.
Actually, if you have a plastic airplane by wenmac that came with the motor, THAT is sure to be worth something. Some of them quite a lot. What plane is it?