The Eton Rascal parts are the closet you can get to the original
Koosko engine parts.
http://www.etonparts.com/rascal_ixl_40.htm
These are in no way guaranteed to work with the Koosko engine, but the
parts are kinda like cousins/descendants of the koosko engine. You
might get lucky?
That being said... not to rain on your parade but a new cylinder and
piston head will cost you almost as much as a new engine. Especially
if it's original Koosko stock parts.
A few people on here including myself have had success with this
engine ($250)
http://www.davesmotors.com/s.nl/c.885035/n.1/it.A/id.4506/.f
It fits the koosko ... but it has a few issues though:
-You need to either get a new gas tank or modify the existing one
since the engine's bracket mounts are in the way of the tank.
-Also you need to find a new way to mount the engine since the old
engine bracket wont fit, but besides the bracket the engine fits very
well.
-Also the spark plug is in the way of the top delron rod, that needs
to be modded.
-Also you need to either mount the muffler it comes with upside or get
a new exhaust.
Actually this thing is a bitch to get in there. But most the mods to
make it fit are minor and can be done with few/household tool's and
cheap/no extra materials.
If you can't find eton replacement parts and dont want to go the gp460
route you could always find an old 2stroke weedwhacker engine and
maybe youll get lucky and it will fit?
Keep me updated and I'll try to help
-Kev