Hi Paul,
No answer for 6 days is probably because control line flyers here usually discuss using the Saito 30-40-56-62-72 so they have no experience with a "50". I never even knew there was a 50.
Maybe, some RC guys may have some experience with a 50 and could weigh in.
The obvious answer to your question is go to whoever is the equivelent of the US distributor Horizon Hobbies in your part of the world. However, when I looked at gasket sets on Horizon's website it was confusing because they use a 2 letter code "AA" or similar but I could not find which gasket fits what engine. So you have to figure out what code letters equals your engine and then check your distributors website for availability.
The other option is to send the engine in for service to replace the gaskets and then it becomes the service centers problem to find the right gaskets. Good Luck.
Pat Robinson