If it were me whether you go for F3 or modified production I would go for the MC21 over the MC18 simply because parts availability (new and used) is far greater. Honda still sell new cranks for the 21 (around £700?) the standard pistons which fit all models are still in production.
One of the problems if you went F3 is new piston and crank supplies are long gone (the NF5 and F3 run 14mm little ends as opposed to 15mm standard) I know Dan ran a standard crank and pistons in his F3 (with the double taper NF5 flywheel reground to suit) because of this and his bike still produced very good power I believe.
Tyga have a good range of stuff and are still developing new parts for the MC21, all their exhaust system and rear set parts are sold individually as well so good for crash damage repairs
Andy might know if they have any other performance parts in the pipeline?
I have a MC21 track day bike which is currently running a standard dry clutch engine with HRC air tray, pod filters and JHA pipes which makes and makes around 59 bhp on Krazy Katts dyno.
Chassis wise I have an alloy sub frame, fibre glass race bodywork, HRC loom (basically standard loom with the road stuff deleted) NX5 forks and brakes, HRC tacho, Tyga temp gauge, NF5 magtek wheels, NF5/F3 rear brake system, Tyga rear sets, F3 rear shock and probably a few other bits and bobs.
I have yet to weigh it but I have a couple of stock NSR's and a NF5 just pushing it around the workshop the weight difference is really noticeable compared to stock but obviously still feels a little heavier than the NF5.