Starter boxes are much better to use & easier. Hobao might still make one, not sure though, I had it for my Hyper 7 PCR, it was an rtr with pull start but after breaking a couple of them I got the box, it was a 12v one though so I had one of them small jump starter setups for a 1:1 car that I hooked the box up to. I think OFNA have a box that would suit but it is just a rebadged version of another brand, think it has 2 775 motors. I actually have to get 1 as it can be set up to suit my JQ Products buggy.
As for PRP, I tried to find some info, all I could come up with was a PRP swift 1/8 scale buggy for sale advert on rc tech from 2004, but not sure on engine in it & an ebay add for a used PRP .21 like yours with pullstart for $50, that same seller was also selling a swift 1/8 nitro buggy. so maybe the engine is the rtr swift engine, if so then you probably won't find any parts unless you can find used parts on ebay.
You might be better off getting yourself a new better engine.