[DonRotolo]

To others reading this post: Note what he says is the distance between cam and crank - one point two-nine inches. This is one very small engine.

Borna, you may get better results (and more informed responses) on a micro machining / engine hobbyist site. Steam engine folks also.

I am curious why you want to put the bigger pulley on the cam - in most 4-stroke engines the cam runs as twice the speed of the crank, not half the speed. Also, you MUST use a cogged ('timing') belt, since slippage will throw the timing off almost instantly. Or a chain.

As for tensioning - a belt tensioner just pushes against the belt to keep it tight, size is almost irrelevant.