I replaced the rings on mine (you can even add dual rings) using the OEM UMS rings. I was able to order new o-rings and cylinder rings for my engine directly from UMS. Wasn't terribly expensive either. This was over a year ago now, so possibly Horizon has gotten wise to this and told UMS to stop it; or UMS has become wise to Horizon not wanting to sell parts and jacked their pricing up. Either or. I contacted Horizon at one point and *****ed about not being able to purchase spares for what effectively is an enthusiasts engine - and when I spend money on something like this, totally for hobby fun, then a company shouldn't be attempting to block me from playing with the engine after I purchase it; especially if I assume the risks of voiding any warranty by contacting them to tell them I don't care about the warranty!

Push a little bit and you should be able to obtain the parts. I have a whole spare engine I have myself for parts as I planned to keep this thing running for a long time. Not sure how to get the head off, but I'd imagine some heat and a pair of solid grips will help separate the two halves if needed.