1991 325iC Engine Options

I've got a '91 BMW 325iC, and have a stock inline 6 cylinder engine with 160,000 miles. The car runs well, but I would like to upgrade the engine. Other than going with a stock new or rebuilt '91 BMW 325 inline 6 cylinder, what are my options? Would it be possible to use an M3 engine of the same year? Anything I can do to a stock '91 BMW 325 inline 6 cylinder to increase horsepower?

if you are using the engine from another car of the same year, or from another e30, make sure you get one from a 325i* car. Other ones have less power, unless it is the m3 engine. You can also swap in a newer m3 engine, one out of a '95 would be the most popular. Another option would be to swap in a standard e36 engine.

Thanks a lot for your input, do you have any idea how much modification im going to have to do to swap in a '95 M3 engine? I know I'd need a new clutch and transmission, but what else?