Depends on how much you can get a crank for......if your trying to do this on the cheap i suggest you leave it as is as splitting an engine to change stroke will require machining and parts, at least rings, bearings and gasket kit. Replace water pump, timing belt, camsafts & gears etc etc. if you then are paying someone else to blueprint the engine you will be looking at atleast $1500 in labour. I reckon to stroke RB20 using second hand parts ie crank and rods, machining and other parts + labour to build.......$5000. Cheapest option would be to get a 3litre and put you head on it.