Can I add a main breaker to an old split bus panel?
How far this breaker box to the fuse disconnect switch as you mention ?
To get the new breaker box the cost is not that bad at all you can get a common 100 amp 20 space panel that can be under 100 Euros ( dollars to your side ) and replace the conductors from the outside main fuse to the inside panel so therefore you will have to replace with 4 conductor cable which it will have a pair of hots , netural et ground ( typically black ,red , white et green or bare in correct secquine as I menton above.
You will have to get the permit for this one I do not know which state you are in so the codes will varies a little.
The answer will be based on NEC ( National Electrical code ) or CEC ( Cananda Electrical code ) or ECF ( Electrique Code France )