Top replacement?

Put the whole roof on or you'll be chasing the rust . I did a 62 years ago and after cutting out all the rust I ended up putting the cowl on with the a-pillars.. I had others where I just cut the a-pillars at the bottom.

I am sure you are going to find the a-pillars are rusted on the inside. Once the rain gutter rusts through the water runs through all the inner framing and the moisture will be behind all that inner structure of the roof where it was bare metal from new.

The moisture evaporates up and usually moistens the whole inner roof structure. Remove the roof skin and you can clean up the frame. But you will never clean out the a-pillars and you will never know how far they are rotted on the inside. Been there.