I have this for sell which I think would work better for this size plane:
http://www.rcuniverse.com/market/item.cfm?itemid=754802

If you want to stay around 40cc this guy has two ZDZ40s:
http://www.rcuniverse.com/market/item.cfm?itemId=753045
My friend used to run ZDZs and they are nice motors.

Brillelli made a good 45cc, but I don't see any for sell right now.

Fuji makes a 50cc, but they are expensive to repair, heavy and not supper powerful for they're size, but the weight and power shouldn't be a problem in your plane:
http://www.rcuniverse.com/market/item.cfm?itemId=753395

I would lean toward too big of a motor, that's what throttles are for, if you have too small a motor you have to buy a bigger one then put it in, never as neat as the original installation.