I grafted the whole CRX50 Polini disc front end (Bars, levers, tree, brake setup, forks, rim & tire) I scored off fleabay for my sons 50. At 33mm forks, its the same front end setup on the earlier model KTM 50 Seniors. Also swaped in the Polini rear coil off the shock onto a fastace adjustable to give it overall the right balance for my 10 year old son. Went for a genuine CRF70 throttle to fit the bars, then all sweet.
2 versions of fork lengths on the KTM's 50 & 65's are about, so shop carefully - longer ones will need some serious rear end balancing - the 50 caliber 3+swingarm is ideal for raising the rear if you go serious like I did.
I bought taper replacement bearings & replaced the seats in the stock XR frame head stem. Some work was required machining to fit the new bearings on the tree, and on full lock the upper fork brace touches the tank restricting full lock turns, but doesn't impact on overall riding. It would be nice to get a 100% fitting tree like a Marchonni but the triple tree costs more than the whole CRX50 I stripped for parts ! I've recently bought a 2003 RM65 33mm triple tree & mock fitted up. It will require a new smaller centre shaft, but with a new machined shaft and spacer on top of the top bearing it will give me the lift needed to clear any tank fouling issues, as well as raise the bars so that any standard MX bar can be used rather than the traditional 6" rise bars.
I'm amazed at the current outcome though - with 9" both front & rear I can ride both my xr600 and my sons xr50 around the same track and both feel near the same. Although the 50 has softer suspension for his lighter weight it soaks up everything without bottoming & without missing a beat with me on board. Scary in fact as you end up at the same speed as my fullsize, but with only 10" rubber between me and the ground it makes me want to backoff !!
I would have no hesitation in recommending the KTM front end swap in