I happen to have 65 sportster that has a "91" on the backplate and it came with a bag of 12" unmarked rev-up props, I never suspected it might be a drone engine. I have not run the engine, but it looks to be a earlier K&B without the bronze bushed crankcase. I suppose this crankcas will wear its shaft bore quickly? i don't yet know what I will do with this engine but I might put it on the stand and run it some. I have some fox fuel w/20% castor oil lube.