There have been some fitted with OS and TT .18 engines,so basically any small block will fit.You might possibly have to trim the fins if they foul the frame.

You'll need a pull start engine preferably with a Tiger drive type backplate.
I did fit a Rossi .12 pull start in one but it required some filing on the frame to clear the plastic pull start,and entailed moving the engine towards the RH side.

The TT engine fits very close to the LH frame and you'll probably need to drill and tap new engine lug mounting holes in the engine mounts so that you can move the engine over to clear the backplate screws.

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