Mercian ‘77 [kinda] new build

I'd go modern:

1. Trawl eBay for an entry level Alu road bike that a weekend warrior gave up quickly on.
   
2. Move the parts over to your Mercian frame
   
3. Sell the alu frame and your spare fixed bits and saddle.
   
4. Post in modern groupset on old frame thread. / Profit.