Nige63 , you have me confused 

good answers Derek996   

Nige63 ; how many wires has your switch got ?

two wires
one to    battery -ve (negative terminal)
one to fuse and then to thorspark

as per the diagram shown by Derek996

this may help (but I doubt it    Derek didn't put circles on his switch  )

saw this today and thought of you 

https://www.facebook.com/reel/363879876620191

its a facebook link . not sure if you can see it

not sure if this is any good
https://histoparts.com/en/tractor-parts-by-brand/farmall-a-b-c-cub-h-m-bmd/engine-parts-by-model/farmall-h-w4-c-152-engine

so what is the weight difference of the piston out of interest 

front wheel laced 4.5mm offset
1 degree camber on front wheel .

mmm 

Wheelnut
we are in North Yorkshire if thats near enough for you .
motorliner jig
http://www.jwmotorcycles.co.uk/
John Warrington Motorcycles.
01653 698 258
john@jwmotorcycles.co.uk
can't do anything till the new year though 

and if I don't answer the phone its because I am busy sorry

ok so another idea
Motorcycle boring bar
vidlink to illustrate  https://youtu.be/CNBsd-iCJvo?si=YH7e6xANesNFzRZY
this could be mounted on the block head bolts possibly using an adaptor plate
I have seen some where they do bigger verions for cars.  if they could be modified or
adapted it would solve the problem possibly
I think the Van norman boring bar max is 5.5inch.
what bore is the farmall :-)

not sure if this helps
The internation 634 tractor was a dry liner engine , I have done a couple of them in my time
back in my apprenticeship days
you can manufacture a draw tool to extract the liner
ours was a big spanner and a long piece of threaded rod and two big nuts
a large dia tube on to top of the engine block with a cross bar with a hole drilled.
and an insert that fitted the liner exactly  at the bottom.
refitting was a hammer and a block of wood
that was a few years ago now 
wet liners were a lot easier to do 

I'm guessing, now we are 100 years on, the liner  may be slightly attached to the parent metal
of the block and this may no longer be an option   

2nd post    stubaker58    well done 

I got a second hand one for £300  we used it for the 3 months we were involved in the project and sold it for £350.
I now have an auto start on my dyno but it cost me a bit of dosh, the Solo was avery cheap get out of jail
and cost very little , but its each to their own.

I have had customers at the shop with starting rollers and vans . such a faffing going on.
then the trolley with the wheel you push agains the bike.
both two man operations or grumpy wife 

then the solo arrived, what a breath of fresh air. I bought one second hand and sold it when we finished the project we were on with.
https://www.solomotorcycleproducts.com/wordpress/index.php/starter/
cannot recomend them high enough
or any such comparable product although the home brews never seem to be as good some how

not sure this will help then 

https://cybermotorcycle.com/gallery/bsa-frames/

here you go 
T140 into A65
https://www.triumphrat.net/threads/tribsa-t140-rebuild-completed.863442/

google is your friend 

well this is my project Tribsa in bits which should be been built shortly
if I get time.
I could drop the engine plates in for you to see if you like
also there is useful link here
https://classicbikepartsuk.uk/product/triumph-engine-to-bsa-frame-tribsa-conversion-plate-brackets/
not sure how relevant it is but you can read the text to help.

nice one 

I read this
**
I should probably own up to being a bit of a wally when buying the bike test road it briefly but enogh to know it was running nicely, ticked over smoothly and went through the grears OK. I picked it up in a van the next week without even firing the bike up and when I got it home as the weather was rubbish I decided to give it a once over including cleaning the carb. So I have no way of knowing for sure if I've caused this or if something happened with the bike in between the test ride and me picking it up.****

then looked at the pictures
can't for life of me see how it affects separate cylinders
but the slide looks wrong in the picture.