If I'm running a gas engine it will be able to run 100LL, lots of places you cant get mogas.
That's one of the other major reasons I am going with Jet-A. It's available and it's not going anywhere.

Ideally I want to be able to run farm (no tax) diesel and burn jet fuel when travelling but there isn't any engine out there that I think is light enough
There are quite a few diesels out there for aircraft that are at the lower end of the power spectrum (compared to what I am looking at (with 500 shp being the minimum)) that should be light enough for your needs. The problem becomes when you're looking for the higher end of the power spectrum the cancellation of the Centurion 4.0 project pretty much screwed us all over.