I donīt thing you got to scratch build it, at least not in extent, only to add/modify some stuff.
You can start with the kit from AFV Club of the same vehicle in 1/35 and add or change some bits with some Accurate Armour products.
Hereīs the kit I mentioned in an online store, I built the Scorpion (almost the same kit except for the storage bins and the main gun) Be aware of the vynil tracks though, not that good but I donīt know if there is any replacement for them:
FV 107 SCIMITAR Hope it helps
Cheers