I would not recomend an engine swap, rather just enjoy the car for what it is.
If you don't want that kind of advice, I would highly recomend dropping in a M52 2.8L w/VANOS. I don't recomend an ///M engine because they are VERY expensive to maintain properly and if you find one out of a wrecked ///M car, chances are the car is wrecked for a reason....because the guy drove the car hard and one day, too hard.
Regardless, with the proper mods and adjustments the later M52 non ///M engine can be massaged to similar power and torque with the bonus of much cheaper parts, and maintenance.
That's what I did, and I am VERY satisfied!!