I'm sure you are aware of this, but just in case......
Planet Hobby
www.planethobby.com has a lot of Novarossi engines and parts. You might be able to compare the header they have for the 50 with yours to make sure it is the right one. I once ordered the 09.20C from them and the header had a nylon insert that mated between the header and the round portion on the head (not sure what you call it, exhaust manifold?). Without this insert you would have the problems you mention although I do not know if the 50 has this insert. On the 20 the header was held on by a spring but I cant remember how it attached. I think it was through two small holes in the header that the spring hooked through. I do remember it was kind of a flimsy attachment. The 20 was a fine and very strong running engine but it was a boat anchor. I have seen a few Novarossi 50's and 60's for sale here and elsewhere and I am tempted to pick one up. However I have a Webra 50 that is running just fine and I cant justify the money for a Novarossi. I am surprised that they are not more popular however. I would be interested to know how yours performs. I have dealt with Planethobby several times and I have been impressed with their service. Their glow plugs are a real bargain in my opinion.
hope this helps,
Rob