Bil Herd has posted a collection of schematics to a computer known as the C64 Plus.
More schematics from 30 years ago. This is for a computer known internally as the C64+ or C64 Plus and has built in 80 column support. Unfortunately there is a page missing.
Check these out on c128.com.
I really wish someone would complete these computers as pet projects.. I’d love to see this or a C65 actually become reality.