??? A trailer to transport your trike? That is very unclear. As to the rest of it... I'm with the posters suggesting electric propulsion. I'm thinking you need 500Watts (or more) and 250W is a waste of time. Especially if you want to increase the wheel size to 26". But a wheel is not an entity unto itself. Wheels function as part of a system with the fork, fork rake, headtube length, head-tube angle... ... you just can't change a front wheel from 20" to 26" without some significant effects on handling. What was the trike designed for originally? A poster made the observation that small 2 and/or 4 stroke engines are smelly and polluting, and this may or may not deter someone who doesn't give much credence to the effect that small things have on the environment, but there is this: small bike engines put the cyclist (if you can still call them that) in intimate contact with gasoline, 2 stroke oil and their exhaust products. They will kill YOU, the user, slowly for sure, but you will die sooner using one than otherwise and not just because you will be missing out on beneficial aerobic exercise. FWIW.