On my girlfriend's D1800 (2wd) the air filter is a normal tractor filter. Take the old one off and go to Napa or your local tractor dealership and have them match it. It shouldn't matter which color/brand. It's the same filter as is on my Yanmar and International Harvester, and I've seen one like it on Deeres and an Allis-Chalmers. I've bought them at the local Kubota dealer.
Hers doesn't have an external hydraulic filter. It's probably just a screen inside the transmission case.
Post in the Mitsubishi forum, and be patient; it's not very active compared to the others, as a rule. Be as specific as you can, too. What's wrong with the driveshaft? Is it missing? Broken? A driveline shop can build you one. I'm assuming it's the front. Good luck!
I do not know what the driveshaft looks like but I would contact a driveshaft maker and see if he can make you one. I have had Carolina Driveline make shafts for me at reasonable cost and very good quality. You may find someone similiar but here is their website. Carolina Driveline: Driveshafts Speed Balanced Steel and Aluminum