i think a nice homelite 25cc or ryobi 31cc would be very nice. they are cheap to convert and dont use hardly any fuel. the 31cc ryobi is nice, if you dont want to cut up your cowl (the carb is mounted on the rear). the homelite is more powerful, but for a cub, i dont think the extra ppwer is needed. the ryobi will spin a 16-8 up to a 18-8 prop.
if you need more info, go to the conversion engines section of this site. lots and lots of info