Yes, a rack battery would work. I wouldn't have any problem converting it myself, but I have the time and determination to get it right. It might be just a case of swapping out the brake levers for the kit ones, or the kit ones might not have the right stroke for the hub ones. Somebody has to try it. Next is the question of chain-line., which doesn't really matter on a bike with derailleur gears. Again, it might come out right, or it might not. Finally is the anchor for the motor that fits behind the seatpost and BB. It doesn't look too bad, but you don't know until you try it. The stand would have to go.
If AllyPally reads this, I'd be happy to do the conversion if they can get the bike to Telford and back. No charge, so the cost of the transport could be offset against the normal cost of conversion.