Is it possible to make my solid wood 1930s door frames smaller for modern 30min fire doors, without having to take out the originalframes.

Yes, although it won’t be easy!

You can

1.      Fit an approved liner to reduce the gaps.

2.      Relpace the stops and add intumescent system

3.      Pack the frame with hardwood and intumescent

However, you can not reduce the opening by more than 12mm per-side

Yes, its possible but alternatively you could have a fire door custom-made, from a certificated door blank, to fit the existing door frame. Either way, my advice would be to seek expert assistance to make sure everything complies with the relevant standards and regulations.