All the parts you need can presently be found new on ebay UK:
Flywheel with ring gear:
http://www.ebay.co.uk/itm/GENUINE-LISTER-LT-LV-FLYWHEEL-AND-GEAR-RING-ASSY-601-51562-/261420028125FYI This flywheel looks as if it the version which incorporates a charge winding. If so it will differ somewhat dimensionally from a hand start flywheel
Starter motor spacer:
http://www.ebay.co.uk/itm/GENUINE-LISTER-LT-LV-ENGINE-DISTANCE-PIECE-STARTER-MOTOR-early-style-602-37080-/251784047251Starter motor:
http://www.ebay.co.uk/itm/LISTER-LT2-LV2-ENGINE-12volt-STARTER-MOTOR-602-37092-equivalent-/261620284130Cast Aluminium flywheel fan shroud:
http://www.ebay.co.uk/itm/GENUINE-LISTER-LT-LV-FAN-SHROUD-SAE-5-FACING-601-37670-/252207751400Though by the time you have added that lot up, as others have stated a donor engine would be another (and cheaper) option to consider. I cannot be 100% sure but the starter motor that I have put a link too, looks remarkably similar to what you would find on electric start Petter AC and AD range engines.
Thinking about this in more detail, electric start LT and LV engines were never that prevalent (there diminutive size meant they were always easy to hand start). Hence here is another potential solution for you to consider (and one that Listers themselves used on HR2 and 3 electric start engines, when fitted with sheet steel flywheel fan shrouds, for applications that required electric start but were not close coupled).
Still retaining the sheet steel flywheel fan shroud, it may be possible to fit a steel plate say 3/8" thick (secured to the flywheel end main bearing housing studs) inside the cowling. This would then allow a starter motor to be bolted to this plate. Finally due to the LT flywheel been a comparable size to an automotive engine, you may be able to find an aftermarket starter ring to fit your flywheel. Finally a suitable pre-engage type starter motor could be sourced from any scrapyard. This is obviously not a five minute operation requiring some machining work, and significant time to source suitable parts, but it would be a lot cheaper than buying the parts new.