I'm don't know of any direct crossovers. I think the wheel spacing, axles, hubs, etc. are unique to GasGas bikes. The rear sprockets from Husky crossover but I would be skeptical if everything else matches up perfectly.
If you are looking for 19" rear wheels for motocross, why not get the wheels from a GasGas MC250? If you want 18", then I would buy another OE set like you already have.