There are four(4) screws on the bottom of the Arc Mini R2 that hold the bottom drive cage in place.
Removing two(2) additional screws from the motherboard tray allows the lower drive cage to be removed; no drilling required.
With both drive cages removed, there is space for a 240mm radiator, up to 272mm in height.
This 240mm Black Ice GT Stealth, however, is 277mm tall, so it does not fit.
The 5.25” drive cage is held in place using screws, so it too can be removed without drilling any rivets.
With all three drive cages removed, there is plenty of room for a 240mm radiator.
There may be difficulty installing an AiO cooler with tubes on the top if there is a fan mounted in the top the case.
With the tubes on the top, the radiator mounting holes do not line up with the fan. (Corsair H60 pictured)
Mounting the AiO with the tubes on the bottom is still possible, with a fan installed in the ceiling.