My advice regarding making a thirty year old cruiser into a sportsbike:
Presuming that you are doing most of the wrench work yourself, expect to put
three to four thousand into the engine (pistons, cams, carbs, pipes, miscellaneous) and be pleased when it
turns out to be somewhat less. Expect to put about the same into more modern chassis parts: at a minimum some
"upside down" forks; dual calipers with at least four live pistons each; wheels with good rubber;
and various adaptors: rods and plates. Most anything from this millenium is good enough.
If it pleases you: tank and seat; other bits.
For this much in expense, expect the performance of a bike costing at least twice as much.
It won't beat the top sports bikes from Europe or Japanese fours, but it will be at least a match
for sports/cruisers displacing half again as much (or more). Imagine how the riders will feel
when you dust them.
I see 100 cubic inch piston sets for late model Harleys for $200. I see high performance cams for the same for 125 dollars out-right (no exchange).
I talk to piston and cam manufacturers and despite sending them a thick packet of my merchant credentials,
they will do no business with me because I don't have a physical store. I try automotive sources and they call up to mock me.
They tell me that I'm a sucker if I ever pay shipping from suppliers. They tell me that automotive pistons' retail price is at least
three times their wholesale cost.
I must do my own designs and shop out CAD programs and drawings to get anyone to talk to me. I get quotes that astonish me.
I must buy pistons in lots and supply my own (or customers') cores for cam grinding - which can take a month.
I pay well over retail of the prices of more modern bikes. I buy in quantity and wait on sales. I mark up a very small percent
and if I don't sell a product within a month, I end up losing money. So it goes. . . .