Yes, the 98 can run on compressed air with no extra parts or modification. However, it needs to be a high output tank since Tippmanns generally run on 800 psi or so.
I used to run mine with a 600psi output tank with no problems. Got a similar project myself, was going to make an SL98, strip the metal down to bare minimum... any problems give me a shout, I've had them all!