Harada Shokai supplies Weber/Solex kits for several Japanese engines.
They can even supply the Webers but I doubt that would be price competitive.
Site is in Japanese but web based translation worked well for me.
So here is an option to vastly simplify use of Japanese engines.
Webers and stand alone ignition rather than fighting all the extra
from an original wire harness.
http://www.jcy.co.jp/hrd/Email is hrd#gol.com (You have to replace the # with @, I guess they despise SPAM too).
No affiliation but I may buy one of their intakes for a Miata 1.8.