531 is Reynolds tubing, primarily a Pommy thing, not sure that it’s available outside the UK.
4130 is Cro-Mo, I don't think it’s not that expensive in the states.
Setting up for a modern motor in old styling should work for the road bike crowd, but if you have to change the frame geometry from the normal Metisse to fit a modern motor in, or just change axle mounts etc, then using the frame for Metisse kits will be compromised.
Too many changes from the norm may make them ineligible for some classes.
Racing rules aside, a lot of kits get built up to never see the track, but the guys that do this usually won’t settle for something that isn’t period. People often customize the cosmetics in order to be individuals but they generally chase authentic, no matter how good a new version may be.
Generally speaking the road riding guys (not road racers) won’t care.
Hey Firko, I have looked at some of your kits that you have bought in and thought they may be good to replicate, but to do them properly would be way dearer than what you have been buying secondhand frames for. Not sure there would be a market for them at new prices.
I guess you worked that out too.