Mike,
Brian from AD Performance
Cylinder Heads : ADPerformance, Your Source for Longblock Performance Parts has both Eagle and Scat cranks and some others. also pistons, rods, heads etc. Very helpful and pleasant to deal with. No problems and was happy to ship my stuff to NZ. Full details of options and prices on his website. May be cheaper and better than buying in the UK. Tell him what you want to do and your budget and he will advise your best options.
Don't forget the advertisers on this site. I have dealt with Keith Craft also and he is very good.
If you want to rev it, it is my understanding that you need a non-crossdrillled crank i.e. straight shot oiling, but I'll leave the technical stuff the engine gurus to comment on. I would definitely go internal balance. External may work OK but it's an abomination of an idea having out of balance forces flying around on the
outside of a motor. IMHO
Cheers