A main lug only panel will be OK, though it may be cost effective in some circumstances (and perhaps convenient) to install a main breaker.

Things to check are to make sure your grounding electrode (rods typically) at the detached building is in place, that the 100A fuses in the feeder are in fact the right size for the size of wire used.

If the panel has more than 6 breakers then you need a main breaker or a separate disconnect at the detached building.

This sounds like an older installation. There are several considerations in upgrading such an installation - for example, the feed to your building may not meet current code grounding requirements.

At the very least, you'll need to be sure the feed is protected by an appropriately sized overcurrent device. A knife switch (!) isn't sufficient.

You may be getting into more than you bargained for here.

i would use a squareD qo main breaker panel

I just left a "knife switch" (enclosed type) in service as a disconnect for a residential furnace.

Charming contraption, original manufacturer (Trumbull) rated it to 30A, it's on a 15A circuit. It's in quite good condition.

I did replace all of the wiring on that circuit though. What a !@#$ working in a box that tight.