Depends on how rough you want
Say you were to source all the separate components yourself for a 175cc 2T air cooled motor .Buying good quality motorcycle, Jap or Italian tested parts. Paying all taxes and shipping you would be looking at around a grand for most the bits.....
The expensive bit and the only part not already mass produced for the motorcycle industry is the crank case. And although its CnC`d from a £80 lump of billet or rough cast, its cost the motor manufacturers thousands of sqid and man hours to design, test, and adjust for the best performance, weight saving, mounting points, re-drive options and structural reliability.
With such a small market to cater for, the cost understandably gets passed on. I`ve not seen any prices or asked. But £700 at least would be a guess. Add final assembly, in house machining of pulleys/mounts, tuning the exhaust, testing, marketing and a little honest profit....That gives you little change out of £2.5 grand
Failing that lob a couple of pulleys and a prop on an old sump pump that's been chuffing for years (and very well I might add) and you have a more than reasonable motor for half the price.
Yes its all very expensive.....but bloody good fun
Colin B