There are quite a few in the "building it is half the fun" space. You've got things like the Omega (MSX) and ZX81+38 (Sinclair), and Tynemouth Software made PE

There are quite a few in the "building it is half the fun" space. You've got things like the Omega (MSX) and ZX81+38 (Sinclair), and Tynemouth Software made PET and ZX80 clones. The RC2014 ecosystem is interesting; it's sort of a more modern take on the early world of passive-backplane 8080/Z80 machines-- you can build it pretty much vintage-only or with a variety of modern parts. If you want to stretch to 16-bit there are a couple of 8088-based kit projects too, ranging from "all old stock parts" to "a few affordances for modernity",