That conversion kit just adds a trigger linkage from the position of the normal trigger up to the 6 o/clock rail. Not very ergonomic for chaging fire mode and releasing the mag by modern standards, but then the L85 is just the same (apart from the safety).
I think that's brilliant to be honest, just take off the stock, clamp on that weird shaped adapter kit (seems to be all one piece).. you're done. I can't really read anything on the kit apart from 'MAG' so googling isn't much use, but that looks like it could almost be a commercial product, be interested to see if it's for sale. If anyone with a WE SCAR-L GBBr could get one of those kits, it'd quite possibly work.