Hey guys & girls
I have an issue with my 81 Datsun b310 not starting

Car history: it used to be auto and a12 engine
Car now: has been converted to Manual, a15 with weber 32/36

I've always had an issue with the car being hard to start as it
Don't get driven a hole lot
2 weeks back while sitting idle in the drive way it just stopped and would
not start by key, everything was working but nothing from the starter motor?
Swapped out starter and relay and still nothing, then a black wire running to
The starter relay cooked its self

I can start the car if I put the dash reds on and jump the starter motor
With a screwdriver

I have checked
Key Ignition switch
Battery
Swapped out starter motor
Fuses
Auto park switch
Earths
Coil
Fuel
Swapped out starter motor relay
Swapped out alternator reg
Fixed the melted wire

And the car is still not starting, everything is working but starter motor
Is just not turning over
Could the car being hard to start have burnt something out?

Anyone had something similar happen and how did you fix it?
need help with this its doing my head in
I have no way of taking the car to the mechanic

Cheers guys