At least here in Sweden, the 240 is certified up 220 hp if memory serves my right. So any engine with less than that is possible, with a turbocharged B230 being the easiest and most popular choice for a swap. The car has to be from 1992 or earlier though, due to EU rules.

However you CAN do much crazier builds but that requires custom brakes, suspension, drivetrain etc in order to register it and make it road legal.

My guess is that most of Europe have similar rules.