Honestly, those year models are pretty much forgotten about. If its not the new thing the aftermarket really doesnt support it. Performance wise you are NOT going to have much luck. Beyond a turbo there is relatively little the aftermarket world supplies. You can find niche communities that build there own stuff and will sell it to you but that is about it. FYI, oilburners.net is a great resource for these engines. (sorry if refering to another site is frowned upon!!!)
The only real aftermarket item is a turbo. Only three companies make them anymore: Banks, ATS and Hypermax. Beyond a turbo you are going to have to fabricate anything you need for performance.
With that being said your best bet is Hypermax. They have a few, few being the keyword, performance parts for these engines. I would look at their turbo if I was going to buy one. Kinda wished that I would have for my IDI.....may it RIP.....
LMC Truck magazines are another source for parts and some aftermarket items.
Hope this helps a little bit.
I miss my 94 IDI so much. It was a pain at times but cheap and reliable!!!