Please correct me if I'm wrong but can 2 litre barrels and pistons be swapped without any mods for 1800s i.e. no machining neccessary to case and heads?

Yes, but you need 2L crank + rods as well.
:beer

if you are after a type 4 engine rebuild try
Conti Engines
Shed 1, 38 Armitage St Bribie Island QLD 4507
ph: (07) 3408 3177

They are building my 2258 type 4 engine at the moment and i know they have a set of 2L rods and crank because thats what my engine started at.