Gear boxes are optional on all engines except 912, which is a combination gear box and starter. There is no designated letter for that gear box. Gear boxes are normally specified and ordered with the initial engine order. All Rotax engines, except 447, are oil injected unless ordered otherwise. The option for oil injection is available for the 503 engine, must specify if not.
Typical radiators are single.
There is an option for a dual radiator, but only for 582 and 912 engines. 447 and 503 engines are air cooled, no water is necessary. They do not require a radiator. All engines have a carburetor factory installed.