Interesting project!
I guess this would be a case of cutting one along the middle of the sleepers of one road, replacing the cut rails with new ones of the correct length, which I would guess is 63-64 mm.
I would secure the butchered crossing to a concrete slab with a suitable exterior adhesive or mastic, filling the centre with a car body filler like Iospon? If G1 sleepers cannot be threaded onto the code 332 rail, I guess it would be easy enough to extend a few spare LGB sleepers to G1 gauge though using solvent weld.
Here's a linky to the service diagram if that helps....
http://www.champex-linden.de/lgb_produktdatenbank_d-e/medien.nsf/medien/DB3ED75307F7024D85256EE0006D233C/$FILE/13100-1.pdf