The alloys won't fit for 2 reasons:
1) The studs are the smaller diameter, so the alloy nuts won't fit, and
2) The studs are not marked for taking alloy rims
... and, as "diesel jock" points out, there isn't a lot of space between the bug & the trailer wheel-arch - and the large inner off-set of the allow will probably foul the bodywork.
(I suppose you could always sling a different axle under the springs, but that leads to a whole other world of pain vis-a-vis brakes)