Most Mitsubishi projectors respond to one of two IR protocols. Proton device 240, and Mitsubishi device 71.
Given the age of the projector, I suspect that the Proton is more likely. Got an Atlas cable company remote that you could test with? Try setup code TV 0868. If you have a Comcast red OK/Select button remote, post back and I'll tell you how you can test with it, using setup code 21278.
The Misubishi procotol setup codes are TV 0150 (10150 if the remote needs 5 digit setup codes) TV 0817 TV 0836. Lots of UEI made remote (like Atlas and Comcast, some Radio Shack, OneForAll) have these setup codes.
The RCA RCRP05B has all of these codes. It is very likely to work, or we can program it to work. Less that $20 at Best Buy, Walgreens, etc.