I run a number of vintage Ultimas. From your question im guessing you want to run your Ultima against modern cars, the Mk1 Ultima is not a competitive car anymore, it doesn't have a slipper and the gear box will not survive fast brushless motors. A 21.5 or 17.5 would suit the Mk1 and make it drivable. in a vintage class with the right motor it can hold its own. I've taken my brushless out now and run 27T stock brushed, geared correctly its fast enough down the straight and smooth through the infield. If you run brushless u want to run 32dp gears as they are much stronger than the 48dp hop ups of the time.
you can check out one of my Mk1's in the vintage section.
Good luck.
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