At Tynemouth Software's Etsy store, you can buy a ready-to-use Raspberry Pi housed within any of number of classic computer cases. The keyboards are properly hooked up, modern outputs are discretely added, and the prices are reasonable.
Pictured above is the ZX Spectrum model, at $308. Also on offer are refitted Commodore 64s, Atari 8-bits, ZX81s, Texas Instruments models and all sorts of customizable bits and bobs.
For about half the price, you can have the old machines refitted for use as everyday USB keyboards. The creator explains the chording witchcraft required to glean sanity from 40-key layouts.
The ZX81 has 40 keys, which have been mapped through to normal keys. I can offer two layout options. The normal one has numbers and letters as normal, and the shift button gives escape, arrow keys, symbols and function keys. The alternate mode is to send each key without mapping. This is better if you plan to use a ZX81 emulator or remap in software yourself. This can be changed at a later date via jumpers.