I would just use the standard rings...
You have to get the cylinders machined and New pistons to go larger....
You can pick up a good 1600 cylinder easily enough at most wreckers etc... Just hone it...
Its unusual for a Power Generator to over heat IMHO...
but I'm pretty sure that Your engine would be a single port 1600 with the early style oil cooler which sits over NO 3 cylinder, which gets preheated air after it goes thru the oil cooler.... this system was introduced in 1938...
and Many VW engines have dropped valves etc with No 3 cylinder... over the years....
they changed this system in 1971 when the doghouse oil cooler was introduced along with a wider fan etc...
the air then goes thru the new oil cooler and the hot air is then sent out of the front of the Fan shroud near the gearbox...
I would just use 2nd hand parts to repair the engine...
although New rings wouldn't be expensive....
Best of Luck.... I did see one of these units a few months ago.. a few of the locals have them...
Lee
http://community.webshots.com/user/vw68autobug
- Helping keep Air Cooled VWs on the road - location: SCONE in the Upper Hunter Valley - Northern NSW 320 kms NNW of SYDNEY---