Hi,
+ 1 on all the previous suggestions,
As you are losing spark strength when you retard the mag, one or two pointers that may help
Very few mag guys have the proper strength magnetiser to remagnetise the mag once its assembled
Some were also fitting sub standard capacitors that are not up to the job ??
Check the points for blackening ? clean them and see if starting improves ?
I think you will find that 3/8in. BTDC is a bit too much with the fuel we have to cope with now a days
I would bring it back to nearer 5/16in.
I have a Super Rocket in a similar state of tune, manual mag etc, I found it runs and starts better at 35 degrees before TDC (if my memory is correct??)
John