Photos would help but as all the actions are the same and its really the stock that denotes the model it really isn't a Mistral any more.
The price would reflect and rely on what stock has been fitted and how well.
If is an aftermarket stock by someone like custom stocks (apparently they still have the jigs) it could be one for the collectors but if a home conversion you can probably count out interest from a lot of the collectors.
Johnj.
Webley Excel, Webley Tomahawk, Air arms khamsin, Air arms Pro Sport, Bam B26-2, Bam B45-3, BSA Mercury, Sharp Innova (licenced Indonesian version).