Really, it depends on what gun you want to do. For me, I wanted an MP5 looking marker. At the time (2006ish?) Ariakon came out with the SIM5 which was a take on the MP5. In the image below it's the top marker.
As you can see though, it wasn't the traditional MP5 look that we're all more familiar with. It had no mag, a funky front shroud, and the solid stock. Good news was it was CLOSE to the MP5 in scale. So, I decided to do what you're thinking about doing. I got some Airsoft parts and went to town.
Let me just say a few things. In the first picture I have my mk 1 mag mod and, up close, it looked like crap. The air adaptor that went in the receiver was huge and I had to bang and bend the mag to get it to fit around it. Far away, like in the above picture, it looked great. Up close it was just azz. So I went with a mk 2 version where I actually had a part made to fit up in the receiver, was much narrower and replacing the stock ASA, and allowed the mag to slide over it without modification. That is what it looked like below.
The stock was another issue. Remember I said the SIM5 was CLOSE??? Well the base of the stock had to be cut off to let it slip over the back of the SIM5 receiver. You can see the receiver sticking out of the bottom of the stock base in the image above. While this worked okay, it wasn't perfect especially since the receiver was also slightly wider than the MP5 and thus the "arms" of the stock wanted to spread out a little when it was collapsed. The base of the stock held them in place but, over time, the stock weakened and broke.
Of course, then there is the shroud. The front shroud had to be cut down in places with a dremmel tool since the paintball barrel is MUCH larger than the actual MP5 barrel. Also, it had to be modified to fit over the slightly larger receiver of the SIM5 too. It didn't look bad, even up close, but a good inspection and someone could see where it had been modified. Also, the flashlight rubbed against the barrel since, in the normal gun, the barrel wasn't so big around. This put some pressure on the light and it eventually failed.
Eventually Ariakon got smart and made a MP5 collapsable stock, a mag kit, and the MP5 style front shroud (minus the flashlight) and you had what looked pretty close to what I had done.
I tell you this because I started with a marker that was supposed to look like the gun I was converting it to. You will have no such luck with the 98 Custom. You will need a lot of time, a lot of patience, and a lot of dremmel work. Your first version may not come out looking that great but stick to it. I've seen a lot of Airsoft to Paintball body conversions and some look great while others fall apart in the first five minutes of play.
Planning is going to be your biggest ally so take your time, figure out what gun you want to make your Tippmann look like, and go from there. Of course, you can also do something that gets you reasonably in the ballpark too. For instance, this is my "Barrett"...
Now, there are many, and I do mean MANY, parts of this gun that are not even close to what a Barrett has but, whenever someone would see it, they'd say "Nice .50" or "Oooh...a Barrett!" So while it wasn't exact, it was close enough and it cost far less than had I tried to make it look identical to the real deal. Something to think about...