The sachs motor is a good one, it has a lot of low-end power, and is very reliable if you don't mess with it too much. There is a place in Italy that has a new cylinder and head for them, but I havn't tried it. do a search on the forum, I posted the link a week ago, and anyway it was just in the links section on this site.
When you get parts from handy bikes, send in your old part- it's a lot easier to match things up. Sachs has a lot of parts that also work for Puch. They had everything I needed. If you get one of the "old ladys" on the phone, just be patient, and try to make them laugh, it makes a lot of diference.
Handy bikes was able to send me a piston that had a little higher deck for more compression, it's lighter, and the skirt will be trimmed to match the old one. If I can det a digital camera, I'll try to take and post some pictures.
I'm getting a spare motor soon, and will be trying to get performance out of it, maby see if there is parts from annother engine that may be close enough to bolt on w/ little modification.