The case of
kidnaper and kidnaped was fairly straightforward, as these variants are much more rarely used everywhere than the double-P spelling.
But as RM points out
worshiped is used quite often in the US, almost as often as
worshipped. (My Firefox spell checker, which uses US English, is querying the double-P version, not the single-P one.)
When a common word is spelled differently in the US and the UK, in conformance with a well known pattern (
fiber /
fibre,
labor /
labour, etc), we class both variants as common. But this situation - multi-syllable verb with an unstressed final syllable ending in
p - is not really a well-known pattern. So I think I have to treat
worshiped like any other word that is not commonly used right across the English speaking world, and class it as rare. I'll do the same for
worshiper and
worshiping.