it probably uses a VW gas tank. which cap depends on the size. Can you go measure it?
Here are some replacement gaskets:
http://www.cip1.ca/SearchResults.asp?Search=gas+cap&Search1=Search
It's probably 70mm
you also might want to vent your tank too (if it's not already).
zig-zag a breather line to reduce gas smell / leaks around corners.