Yes, the owners group at Commander.org is VERY helpful. There are some parts or damage(bird strike) that could be a pain to get through, but most can be handled easily. There are some salvage yards, also some parts cross reference to more common makes, Piper, whatever.
There is a shop in Vermont that specializes in the Rockwell Commander aircraft, Aerodyme. They work only on the Commander and can coordinate with a shop elsewhere with the work. They specialize with a bunch of 'parts kits' and STCs for the aircraft.
Yes, the stock 112 is a big doggy, with the weight, size, & planform being pulled through the air. The IO-390 effectively adds 35 HP, that should suit me fine. At a reasonable power others reported a steady cruise at 140 KIAS. I'm at two or less onboard 90+% of the time. I also gain 42 lbs load with the rear seats out, easy to go.
There are no plans to make new Commanders anytime soon. A Chinese entity has bought up the plans and tooling from the former factory. The whole lot was auctioned off 5 or so years ago.