Hmm, as you say, not that many around. Probably for a reason. I have 2 - they're heavy, loud, slow to re-cock and the Ni-Cd battery (even when new) didn't last long. That said they are part of the Browning/Rutten history, are beautifully made and have their place in history. You certainly think carefully before your shot.
The board itself is quite simple, so it should be possible to repair it or build a new one from veroboard if the tracks burnt. However, due to their rarity, the broken gun may be attractive for its gearbox, in particular.
It'll be too heavy as a wall hanger for most modern homes.
As a breaker I'd say £50-80 may be fair. You'll only really shift it to an existing owner, I suspect. Or as one helluva project gun. Otherwise I fear it is worthless...