Dizzy is a better unit but may have a different map. The starter is fine as is the alternator and most other stuff.
Not familiar with the 3 speed auto but the crank is standard, the flex plate has an adapter which can be a swine to remove, held on with 6 cap headed screws, after that you need the various bits of the flex plate assembly, rather than guess wrong I'll suggest you look it up somewhere, it's been a while since I looked at one.
From memory though I think there is the adapter, starter ring then a hooge spacer followed by the flex drive plate and a reinforcing ring.
Ashcroft's now make flex plates from spring steel, worth fitting a new one.
I did have a spare one somewhere but if I ever get on with it I'm converting the LSE to P38 4.6 spec.