Where is the best place to buy spares for a 503 engine? Are there any pattern part available or only genuine Rotax parts?

I need a set of Pistons & rings, engine gasket & seal kit.

I lost compression on one cylinder, so stripped the engine down & what a mess. Piston has started to break up. I need to get a rebore and a pair of oversize pistons. The good piston is 0.20 over size, I cant read the top of the other one but assume it will be the same.

I will also need a second hand head for the PTO side as this one now looks like someone has been there with a hammer & punch for an hour or two. Lets just say it is far from being as smooth as it should be

Luckily the cast iron liner doesn't look too bad and should be ok with a rebore "dependent on how much over size I can get pistons.

I think what has happened is that a ring has probably been stuck, then broken up and a bit of it got on top of the piston and made a right mess.

As I have to split the engine to make sure there are no bits in the bottom I shall fit new crank seals & check big end bearings ect. There doesn't seam to be any up/down play in the pistons so I think that is a good sign.