parts for the H50 like a billet rod is nearly impossible to find. that's why i stick with the HS50. the motors are easy to come by, as they were used in zillions of snowblowers. And Arc makes a billet rod for the hs50.
you can get an HS50 custom cam too as there's 3 companies that make them, all are good. Most people go with Dynacam, mostly because it's a name brand. but the other two companies are quite good also, just not as well known. I just sent Randy at Dynacam twelve HS50 cam cores, and they welded and cut their 255 onto them. Most people go with 245, but figured why not get the most i can out of it, and went with the 255. You can't really go much bigger than a 255 without modifying the head significantly.
The other advantage to the hs50 is there's lots of info on that motor and how to modify it. In comparison there's very little info on modifying the H50. Also just the size difference between the two motors is striking. The h50 is a monster in width, where the hs50 fits much better into nearly any minibike format.