engine for spotty teenager

[robo]

My mate has decided to copy bolt for bolt what i am building and now has just placed an order with phil for a chassis for his teenage boy, so now there are to be three. I am not going to say what i think till i have had a lightening conductor fitted, anyway i am faced with trying to source an engine 1 litre or smaller that can be swapped easily with something bigger at a later date. Whats out there that will take a rwd gearbox and side engine mounts.also that has a bigger brother in the line up for later on???????????

Bob