Welcome aboard ! I hope we can assist You; & I think we can; because your problem is a common one, when auto transmissions, are replaced with a manual gearbox..
The manual gearbox has no interaction with ignition system, but the auto transmission does. The auto transmission has an interlock contact, which only allows you start the engine, when it is not in drive.
You will have place a bridge, or connect these two wires together permanently, so that your engine will start. There is a thread on this forum about it, & I'll look it up, & place a link in my reply here to you.
https://www.rollaclub.com/board/topic/58879-how-to-convert-an-auto-to-a-manual-ke70/#comment-590797
Alternatively, you can take the "earth/ground" side wire off the start relay, & connect it permanently to the chassis. The start relay, is commonly down on the chassis rail, in the engine bay; directly below the carby.
Cheers Banjo