Growald, if it turns out you have to drill through frame it's not as difficult as it sounds. Most frames are mild steel and with a good quality bit you can go through it fast. I've done it a few times and it was easier than expected. Frame rails are usually 1/4" or less in thickness.
Regarding bolt pattern, I would suggest that if you can't get an exact match, then it would be best to ensure the new hitch has bolt pattern that is different enough that you don't end up drilling new holes right next to old ones. That would concentrate stresses which you'd want to avoid.
Thanks for updating on their reply.
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