It's not so difficult if you had one with insulated wheels, the main issue is you'd have to replace the boiler with a dummy one to make room for a motor and gearbox. Things like the pistons would have to be modified, prob by leaving just the rod, so that they'd work using normal oiling as you'd lose the steam fed lubrication for the piston disc. Axles etc can be oiled normally. You just have to remember it would need regular oiling as it's all metal surfaces compared to LGB's occasional need.
I've considered it but it's probably cheaper to get one built in plastic by someone on a suitable chassis. There's been a nice one done on a Stainz chassis and one of the smaller A class on an Otto chassis, I've got a half built A class in 7mm O16.5 on a Bachmann Porter which is similar to the Otto.