[68Timber]

The easy way to do either would be to rob the sheetmetal that is designed to accept either cap and weld it in your fender. Then the cap will screw on. But a motorcycle gas tank will likely have some curves around that filler that will have to be flattened and it'll be more difficult to do around the mounting flange. Hence the Challenger suggestion. But I don't see why either couldn't be made to work with a little hammering welding and grinding.