I love this project.
I spent a small fortune on a RWA 73 1275 Midget in the early 1990s - including a whole new body shell and a 1293 racing A-Series which was superb. Not so superb was the low (17 mph/1000) gearing in 4th - not eco friendly but good for mullering the occasional Suzuki Swift GTI around at the time up to 80-ish
and of course 'drifting' the odd roundabout.
There are quite a few K-series conversions over here - the K-series goes from a 1.1 8v, 1.4 8v, 1.6 16v, 1.8 16v and 1.8 vvc - power from 60hp to 160hp. Gearboxes vary from ford (Sierra 5 speed - aka Murkur), toyota (Celica - when RWD) and Rover (77 5 speed from the SD1 and MG RV8, Discovery etc.).
As for your engine - its an interesting idea. I kind of wondered about following down the line of the K-series conversions but going for the smaller ones - e.g. the 1.1 4 cyl 8v version (60 hp) which would have matched the original A-series 1275cc in terms of power and torque but much better for economy and much lighter.
Try
Frontline for K-series, suspension, 5-speeds etc.
Good luck.