I just bought a 1984 BMW 318i, I want to know what people might think is the best engine choice for this model?

I bought a 1984 318i BMW, and i LOVE it. I hope to restore it, but I don't know where to start. I know a little bit about cars but not as much as I wish did, so any advince would be very helpful. However the main question I have is, How big of an engine will fit in this frame?I just bought a 1984 BMW 318i, I want to know what people might think is the best engine choice for this model?A new engine int he car is going to be worth more than the car alone...that cars not worth restoring since it holds no value on the car market...it might in about 10 years or so, but right now, its really not worth it...and btw, it would have to be a fairly small engine because it had a very small, boxy frame