GMX: 50HP out of a full on 250 is asking quite a bit! That said, i think a good direction would be ERic Gorrs 144 bigbore kit. The thing is, if you look on the dynos the 167 wont rev out that good and doesnt have much more power than the 144's etc. a well setup 144 is good for 36-38 HP with a very useable powerband. The other problem that i dont understand with the RPM strokers is how he gets the port timing set up well- since he takes the material from the head to make up for the longer stroke and uses a short reach spark plug and if you read any posts here on port timing you will see that this is the most important factor for engine performance and i cant see how this method of "stealth" stroking gets anywhere near ball park port timing. Also, 167 is a lot to ask from a rm125 considering reliablility and the somtimes suspect rm gearbox and the fact you will be dumping in nearly $2000 and the bike wont be worth much after a season or two- a 60mm piston is a lot for that cylinder to take!
that said, eric can build you 155 kit with 58mm piston and a 2mm stroke for a decent price and i suspect it will be way more reliable than a 167! Eric is using a slightly shorter rod to make up for the longer stroke and keep the port timing well set up, or you can use the stock rod and a 2mm spacer plater under the cylinder (ie the stroker kit is visable). (last time i spoke to him he was building a 2002 rm155 for testing purposes)
the 144's do need a bit more attention than a stock bike but they are reliable and very, very good fun! in addtion, since you arent changing the stroke and port timign excessively both the stock pipes and ignitions work well which isnt allways the case on the stroker. have a look on the forsale forum, i know someone that is selling a bolt on 144 kit;)
if eric gets to read this (ie ever gets his PC hooked up) he has my appolgies for answering on his behalf- but i think i covered everything!:)