If you convert to Low Pressure Air youll need to mess with a kit, but if youre using a high output regulator youll be fine.
If you have the Model 98 or the first generation of 98C where you have the plastic rear sight there isnt much in terms of upgrades since that was all pre-cyclone and Egrip. Only thing you can really upgrade is your barrel.
If you want to get a newer 98C stock you can etch out a top groove using a Dremel 84922 bit to allow the stock groove to sit. I just did that on my Model 98 last weekend and slapped an Alpha Black stock on it.