None of the new mags are a drop in replacement sadly, you will have to modify a mag to use in the gun, don't mod the gun! I can send you pics of how to mod a new BSA mag such as this one:
http://www.airgunspares.com/store/pr...-New-Magazine/
Although the index system can be removed from the gun to work with one of these new type mags, it means the old (original) mag you own will no longer work in the gun. Modding the new mags isn't difficult, it preserves the gun in original state and also you can swap between new and original mags as and when you decide rather than having to rebuild the index system every time you wish to use an old mag.