maybe a 15cc eng like the one that just sold in the market place
http://www.rcuniverse.com/market/item.cfm?itemId=719869

you will have to look at your planes and measure from the firwall to the front opening of the cowl. to make sure its not to long.
if it is your best bet with that size cub is a four stroke 70 or 90. a 70 would be plenty. to much engine and you would tear the plane apart in not time. remember its a scale type plane. also the front of the firewall and behind it would have to be renforced. with some aircrat ply. it might be to much truble.. but 15cc is the bigest i would go that would fly a 1/4 scal cub no problem.

Hopefully someone has done it already here and could help you better

try youtube on 15cc gas rc and see what the have them in