I have 3 buddies who are fanatics of the German marque . They have been gathering parts and bikes for years , to have spares , but most parts are fairly available from some shops that specialize in them . Ebay and Mark's Vintage Swapmeet are great sources too . The early 70s 125 and 501 models are rare , collectable , and $$$$ . Don't know if you are thinking of vintage , post-vintage , or both ( My suggestion so you can ride both days at AHRMA or AMA vintage events ) . 1 buddy is a lurker on here ( hey Scott ) and word has it that he may be letting go of some of his fleet ( vast ) , especially the vintage ones . 490s are awesome but have starting issues as do many of the old big bore 2-strokes .
Pentons don't seem quite as popular with the racing crowd but since they originated in Ohio and there is the Buehner's (sp?) shop near Cleveland that specializes in them , they would be a good choice . I had an MX6( 5 maybe ?Had the orange bodywork . ) 250 that had an awesome motor compared to any 2-stroke I've ever ridden . I still have quite a few parts for a Berkshire 100 that I occasionally put on ebay . I rode a brand new 495 KTM in 82 or 83 ( memory is going ) that a dealer friend let me try . What a beast ! More motor than humanly necessary and felt like a chopper with the front end geometry . The Pentons handle different than the Japanese iron and don't suit everyone's taste . Maicos were known as the best handling Euro bikes .
There are specialty shops/dealers all across the country for almost any bike you've ever heard of . Most of them are very helpful but a few are quirky to deal with . Online research and the vintage chats will reveal who is who . There are some companies popping out new and repro parts and accessories for the old bikes ; stuff like pipes , plastics , stickers , shocks/suspension components , seat foam , etc. .
When I decided to get " back into " vintage bikes , I narrowed it down to Penton or Husky and went the Swedish route . From the late 60's on , I've ridden or owned almost every locally available brand of bike , Jap or Euro . Viking biking is to my liking ! Still have my Montesa 360VB that I'm the original owner of and WILL restore one day though .