[Beech]

Try changing to a new bulb in the brake light.
When you have plastic off (watch where short and long screws go), you can check which cylinder is missing by starting the bike up and quickly checking which header pipe is cooler than the others. Infrared temp reader good to have. Good possibility it is a coil. They can get battered by not using the proper tool to pull them.
YouTube is helpful sometimes. And plenty of info on this forum via the advanced search function found by opening the "more" upper right in title menu bar.
and use proper motorcycle coolant to refill, Pentofrost NF is a good one.

radiator removal and replacement:

 

[Beech]

The LED vendors lie. Most will not work with the bmw computer failed lamp detection without an adjoining resistor. If you spend some time searching LED lamps on the forum you may find ones that work.