O.S .90 AX. Just kidding, Just about any .46 sized engine will work fine, just make sure to get ball bearing. Like a few have said the Magnum .46 engine on sale thru Hobby People is a steal. You can't go wrong with it. Just don't get in too much of a hurry to fly and do a proper break-in and it will be a good reliable engine. I already have 2 ASP .46's or I would snatch one up too.