Hi Jon. These days the only way you will keep the original donor's identity is to use the unmodified chassis. Framework can be bolted to it -- but not welded, but it cannot be changed in any other way ( a lot of beach buggy's are falling foul now because the majority of them are SWB and the chassis has lost 18" (so, been modified) ).
The problem with anything mini based is that there is no chassis, being a monocoque design. As far as I know the points system still applies to vehicles using the unmodified chassis (so you have enough of the original vehicle to retain it's identity), but cannot be applied to a Freestyle, which will have to go through an IVA.
Regarding frame numbers -- it has to be a unique 17 digit number, which VOSA can supply, or you can make one up yourself. Mine was a combination of date I bought the frame (second hand so manufacture date was impossible to find), the engine size and TA for Taunton, filled in the zero's to pad it out to 17 digits.