You can get the power pack parts for under £40 per plane on Hobbyking. There’s pre cut firewalls on EBay UK for really cheap if you want them, minis and larger ones I think. PM me if you want the link.
If you want to stop the BBQ skewer holes from wearing out all you need is some 12-15mm cardboard circles with a 3.5mm hole in th middle, glued round the hole.
@mayan has a great picture of his somewhere.
As a former Mini and Morris Minor restorer I love a good SU carb. Not a fan of the bloody SU electric fuel pump though!
Reliant Robin “PILOT” surely?