Aircooled.net sells
a set of 96 mm KB pistons for $275. I believe that you can have your cylinders machined for the larger pistons for about $150. You'll want a
good cam and matching lifters for around $300, cylinder head work for, say, $500, grind your crank for $50, induction for $1000, exhaust $600. Add another $1000 for miscellaneous and shipping and taxes and you have approximately $4000. I don't know if you could do it for much less than that.