If you have a water cooled 250, it is an 85 or newer. The 84 250's were air-cooled. The 86 250 is the first Maico with a power valve. I had an 86 back in the day.
I believe Wossner makes pistons for the 83 and up Maico's. Contact my brother Rich at Pinsonnault moto-X at 413 547 6500. He may have clutch plates in stock.
The 90's models started using sintered friction plates, and steel drive plates, which is the hot set-up to get if you want a good working clutch, but they are expensive. I have a 93 320 and it came with the sintered plates.
The original friction plates in the 83-90 Maico's are the same as the friction plates in the late 70's early 80's Husqvarna's, 250 390 430, and those are cheap and easy to find on ebay.
Email me if you are having trouble finding new plates, I have lots of great condition used.
Paul