[skidlids]

Conversion of a early 583 engine to bigger valve heads isn't that simple as I understand it, If you have a late 1999 to 2001 583 Monster engine then you can fit the 620 heads and domed pistons, To take it to a full 620 you would need the crank as well.
There is a head on Ebay at the moment for £90, so for a pair of heads, barrels and pistons your probably looking at £250 to £300 providing you have the late engine, by the time you look at adding a crankshaft you may as well be buying a 620 engine (earlier ones are 5 speed later ones 6 speed) prices vary depending on age, mileage etc.
If you could run carbs then you would just need some jetting work done to your old 583 carbs.
The alternative to carbs is fuel injection in one form or another, the original setup with Injector bodies, higher pressure fuel pump, coded clocks, ECU, keys and Harness will set you back a fair bit
There is an alternative of going with a non standard non coded ECU such as a FIM module (not cheap) and you still need the harness for the 5.9M type ECU, throttle bodies, pump etc or you could use a 750SSie ECU with a harness from either the 750SSie/900SSie or the M900ie along with a power commander, pump and throttle bodies

Most people are already runing 40mm pipes so no real cost there.

Then again for those wanting to move into class A there are bikes like Trouty's out there for £2k, you can alternatively buy a standard road 620 for less than that.

Going the carbed 750 route may well have tyre implications as well as the age of the engines and spares availability