What size engine??? You can put practically anything you want if you got the money. E36 M3 straight six, BMW 4.4 V8, BMW 5 L V12. Hell, if you wanted, you could put a LT1 corvette engine in your E30. You just have to remember about weight distribution. You dont want your front end so heavy. The most popular choice is the E36 M3 engine. like i said, you just have to have the money.