You might try turbochargingdynamics.com
This link -
http://www.turbochargingdynamics.com/tcd_catal.htm will take you straight to their catalog....
They are fairly responsive and have a few kits out for BMW's and the E24 in particular......There aren't that many true kits outside of custom still out there that I have seen for the E24...Kind of long in the tooth.
Depending on the health of your engine you could have fun with higher PSI boost, but I would not recommend it without an intercooler....
P.S. - If nothing else use the above as a reference point for further hunting/comparison.
Happy shopping.....