Well HM is basically a pretty decent quality bike. Parts are scarce. Almost nothing at all aftermarket or high performance. They won't run with the Cobra's and KTM's but they are pretty darn fast. I'd say its a good entry to mid level bike. Just find someone you can order parts from before buying one. ALL the minis break so I usuaully reccomend buying whatever bike you have local support for. Cobra has a trackside support program at a lot of tracks but KTM is stepping up all throughout the country. In the east Polini has good support but it hasn't made it in the west as much yet.
Its definetley a step up. Much better suspension, about 3 times the horsepower. Only one gear though. Don't fret you kid will pick up riding it quick. One tip with the 2-stroke mini's Change the gearbox oil EVERY ride. WE use the $4 a gallon Dexron3 automatic transmission fluid from Wal-mart in My sons Cobra. But change it every, and I mean every, time you ride. It will save costly clutch repairs. Its by far the most neglected thing on the bike. Also clean the air filter pretty much every time. I have 6 filters and i just rotate them through. We race 2 moto's and then change filters. Sometimes after one I will change it depending on the track conditions.
The Cobra and HM are fairly similar. If I remember correctly HM stands for Honda Montessa. It was a colaboration bike. In the back of the motorcycle mags there is a place that advertises that it has parts. We race with a guy that used to race one. I'll ask him where he got parts from.
If you're looking for a bike for you son just coming off a XR50 I have a 2001 Husqvarna CR50 Jr for sale. I got it for my son just coming of a PW50. It has the same Motor Morini engine that came in the air cooled KTM Pro Jr but has a front disc brake that the KTM didn't have. You can find parts everywhere for these engines. I will also include an extra cylinder, head, carb & throttle for the bike. I also have some of my son's gear that he outgrew that I will include as well, Thor pants, jersey, gloves & boots. If interested e-mail me at: firstname.lastname@example.org
May be a bit late coming to this one but I have (or my 5 year old son does) the HM50 CRX Junior Competition bike running about 13bhp. We initially found problems but now have a really decent supply of parts. If you have one, now, I have some set up tips that were given to me. Change the springs in from 13 on each arm to 11. This will allow the clutch to spin out quicker and give you a better response at lower revs. Also if you are not using the full top end of the bike/engine change the gearing ratios to 10 front and 50 back. The combination of the springs and sprockets will give you a much better response and my son, is flying.
it was a honda montessa colaboration. uses a morini S5 motor. most of the engine parts from a 1999 ktm sr will work. there was a guy in new mexico that did air cooled only ktm's...he was having health problems but he had the best stock of morini s5 parts around.