With a M-98, the one with the gills, you will have to cut the trigger guard to add a two finger trigger. Most barrels should be ok. I know there were minor tweaks over the years on the barrel threads. An older barrel may not thread into a newer 98 Custom, but a newer barrel should thread into any 98 variant.
If you want to install any rate of fire modifications that is where you really need to pay attention. Depending on what 98 variant you have you may or may not have your choice of upgrades.
For the original Model 98 (with gills), the best ROF upgrade that is the E-bolt kit. The E-bolt replaces the hammer with a ram that is controlled by a pneumatic solenoid.
For the 98 Custom it depends on what type of sear you have. If it is a pull type sear then the best ROF upgrade would be the Response Trigger. If it is a push style sear then you can use either the RT or the E-grip.
For the Custom Pro and the Platinum Series you can use the RT or the E-grip as well.