XS400's were never that good, popular or common same with the XS500. Your better off with a XS650 or a TT500 or a RD/RZ 250/350 motor i reckon. My personal pic would the the RD/RZ 250/350 as theres stack of parts and knowledge to make them go good, plus i favour the 2 strokes
Getting an old 4T road bike motor from a wreckers which will most likely be shagged anyway will need a full proper engine rebuild anyway which will not be as cheap as you think, so if money is an issue the 2T single would be the way to go or the 2T RD/RZ as you dont have all the 4T's top end stuff to do like heads valve seats, valves, bearings, cam chains and associated parts etc. Even if you cant get a 490, maybe a DT or MX 400 engine might fit.