To answer the OP's original question in its current state I think you would be lucky to get £250 for it. It is quirky without a doubt but its current condition doesn't make it 'collectable' and any improvement would cost money. As for swapping stocks I really don't think it matters.
My only experience with Stalker rifles is my ownership of a Stalker Jaguar .22 FAC (based on the early Daystate Huntsman)with a lovely left handed stock. From memory I sold it for £400 losing a small amount of money in the process because I didn't use it very much and always reached for my Rapid.It was however an excellent and very accurate rifle.
As a side issue the OP states that the rifle has been taken off FAC and is now 'legal limit'. I'd like to know exactly what happened here because there are some who assert that 'once FAC always FAC' so a potential buyer should check the actual status of this rifle via the Police computer. My valuation takes this uncertainty into account.
'It may be that your sole purpose in life is to serve as a warning to others'.