Transmission OEM builders in no particular order:
TMZPerformance - Tim Zimmer - https://www.tmzperformance.com/
TRE - Jon Ripple - Team Rip Engineering- http://www.teamrip.com/
Parts OEM suppliers and or upgrade options:
JNZTuning - see above post - https://jnztuning.com/
Kiggly Racing - https://kigglyracing.com/
Upgrades and restritors
Magnus Motorsports - http://magnusmotorsports.com/products/
Electrical parts specifically OEM replacement
Sheridan Engineering - http://connectors.sheridanengineering.com/index.htm
Throttle body rebuild:
TMZ Performance - https://www.tmzperformance.com/services/throttle-body-rebuild-services/
ThrottleBodies.com - http://www.throttlebodys.com/DSMTBs.htm
International Mitsubishi parts middle men:
Amayama.com - Singapore based - Shipped direct from UAE and or Japan - https://www.amayama.com/en
- OEM parts supplier direct from Mitsubishi
PartSouq.com - UAE based IIRC - Shipped same as above - same OEM Mitsubishi parts - https://partsouq.com/
Off the top of my head, these were the best options I could think of and their work is beyond reproach. Feel free to add more to the listing, as history racing means you probably need vintage style and OEM exact replacements or period correct style parts to complete the look and function. I have used many of the vendors above, including the overseas parts suppliers for direct from Mitsubishi warehouses to your front door kind of processing/delivery, or shipped from North America to where ever needed.
If using an idle speed controller, please consider that Mitsubishi redesigned and replaced the 1G brown ISC for the 2G black ISC motor mechanism, and these had fewer failures resulting in burnt ISC drivers/ECU's. All of the black ISC's appear to be compatible but a wire change occurred so a comparison and swap off a couple pins is required.
Not sure what diff lockers/ATB units you are allowed to play with, but ATB type units from Quaife which is mainland UK/Britain, is currently working flawlessly in my FWD using an AWD 25 spline units that would fit an AWD Galant GVR4 front trans. Lifetime warranty on most of their units if not all, and no clutch plates - but again not sure what you can use. IF you want lockers in point stages and accel/decel changes in locking characteristics, you are beyond my knowledge base, but I am sure there are some guys who could point you in the right direction. I know we have a member that rallies here that had such devices in his car, but his screen name escaped me. Possibly using Kaaz units front and rear.