How hard (moreso costly) is it to put a modern drivetrain on a vintage mtb?

[winjohn]

<Ducks for cover>
If you really what it to ride and not just to look at then why not just get a second hand newish gravel bike?

Something like

https://www.gumtree.com/p/bicycles/gt-grade-men-s-gravel-bike-size-55/1487354330
or

https://www.ebay.co.uk/itm/23574640...khM80MkWBfc9F097wFmb2zK8k=|tkp:Bk9SR5y0-5XKZA