in my opinion, get h section rods and standard rst pistons.
won't take much to get to 200bhp.
or you could have a zvh sorted to kick out 250bhp pretty easily for little money, like mine for instance without all the extras i've thrown at it.
but if you are keen on a 1600cvh, then about £600 will see it with h section rods with arp's and honed, re-rung and assembled i'd imagine, strong enough for 300 horses then, just needs port work doing and a bigger turbo (stage2 or 3) and efi to get it right.
efi really makes a big difference though.
my old cab was ok on mfi but my xr2 on efi is so much better and more responsive.

also don't forget a better IC, front mount will make loads of difference and crossover pipe isn't really restrictive, but if doing a front mount IC, then you need it modifying to enter from the otherside of the car, they are not a big issue over the engine in regards to heatsoak or flow.