AIM and Century both sell surplus rifles "as is", and it is up to the purchaser to determine if the headspace is acceptable or not. Century could never make enough at $159 assembling the rifles from a parts bin if such a thing existed.
Springfield Sporters is the only big parts distributor, and buying parts to build an Enfield would probably cost $300+.
Another point, the SAAMI gauges are not equal to British Milspec headspace gauges. A SAAMI Field is approx. equal to a Milspec No-Go. Keep in mind that the Enfield headspaces on the case rim, not the shoulder. If you want to have some real fun, measure the rims of various manufacturers cases. You will find a wide variance in the thickness of the rims. I have never found one that will come close to the thickness of a SAAMI Field gauge anyway.
Just to add, non matching makes the rifle a LITTLE less valuable as a collector piece. It does not lower the shootability. Bolt bodies were replaced at FTR when necessary, from servicable parts or new parts. I have 3 non matching, and all my rifles are great shooters.