Hi Danny, it's not hard to convert back to a twin, but mostly I'd think it comes down to how much you are willing to spend (as always).
You would need the complete motor (including engine electrics, carb etc.) and primary drive plus cases, engine plates and exhaust.
I have learnt all this recently (see KH post)..
You might be lucky enough to find a good motor, but you would expect to have to do some work.
The A10 is basically the same motor as the Ariel. I couldn't list the differences, but I think they are minor / cosmetic. Engine internals are the same.
A huntmaster would be worth more than a VB, so you will recoup some of the cost later - plus what you get for the VB motor.
A good Huntmaster will apparently do over 100mph. The one I had wouldn't, but it did go a lot better than a VB - or most singles for that matter.
On the other hand, VB rebuilds are quite cheap. The big end is easy to get and cheaper than one for a VH, ball type mains (also cheaper) - pistons and rings are more or less given away on ebay, as are valves.
You could always have a go at tuning the VB?
'51,'56 Squares, '48 VH, '27 Model C, R67/2, Mk IV Le Mans, '06 Super Duke and Ariel projects.