As far as I know there is nothing to stop the Vikings going on the G-reg, except the problems that currently are preventing most of them from flying.
You mention replacements. The Vikings are fundamentally Grob G103A Twin II Acro gliders. Unfortunately I can't find what their lifespan, but for the K21 it is 12,000 hours which can now be extended to 18,000 hours. Where I fly they fly about 500 hours a year each, and we operate and instruct every day it's flyable except Xmas day. 12,000 hours has taken about 25 years to accumulate. I can't imagine the ATC gliders were flying anything like as many hours, I doubt their initial life was much different to the K21, so whilst the Vikings were built before 1989 I suspect they should have many hours left before needing life extension or cutting up, depending on if there is a suitable scheme of work for extension. Yes, thought is needed about replacing them, but there is no way they should need replacing for some years to come.
Does anyone know how many hours the Vikings usually have, and how many a year they might do?