Well, technically at 100% efficiency 3.5 HP is capable of creating 2.6 Kw. However in the real world, those figures are not obtainable, so you would be best to shoot for around 1.8 Kw max. Trying anything bigger would most likely stall out the Lister.
As for design I'd go for a large circumference alternator because the larger the "circle" the higher speed (fpm) of the magnets passing the coils. I would definitely design it for 3 phase, delta.
The listers seem to be more efficient at a lower rpm than most other diesel engines.
From this you need to determine what voltage you wish for the design to maximize the efficiency, and also remember to figure in how large a battery bank you're charging. You don't want a high rate of charge that will boil your batteries!
