I agree with John that the L1 is a good starting point for producing any of the larger GSWR/GSR 4-4-0's. A number of people have made good representations over the years. The main drawback is that the loco is too long but shortening it is a major undertaking involving a new chassis. If you're going to that amount of trouble, then there are always the smaller wheeled versions, the 333 and 342 classes.
Incidentally, there always seems to be a fair number of L1's for sale on eBay.
Stephen