A single Buhler motor is simply incapable of drawing 4-6 amps (unless it has already internally failed). One of the basic Massoth decoders (such as an emotion M) would probably fit.
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Wont fit. I used Zen decoder that is smaller than Massoth M.. The whole thing is gearbox, there's no inside free space. Motor can easily pull couple of amps, I have seen that figures by just pulling long trains on slight incline. Idk what is stall current of these motors, but I prefer 3+A decoders just in case.
Regardless of current I am not aware of g-scale decoders small enough to fit inside lgb gearbox. HO/OO probably fit, but they are all rated 1A peak.