A quick 2 minutes with a calculator shows that stroking is a lot of work for not a lot of gain with a 54mm piston. Moving the crank pin 2mm further out (much easier said than done) only increases stroke by 4mm giving 268cc on 54mm pistons.
The bigger the pistons the better (hence use on YPVS with 66/64/68mm pistons) - If you use Tyga 300cc pistons (which I guess are 59mm) then you get 320cc on a 4mm stroke.

Stroking is specialist work, and I only know of 2 places:-
http://www.faliconcranks.com/crank_svc.html charge US$868 to stroke a twin, if they feel it can be done
http://crankworks.com/index.php/motorcycle/motorcycle-services-2 they want US$600, again if they feel it can be done.

These figures assume the crank is amenable to stroking - integral crank pins with forged webs are more difficult to stroke as there is not enough metal to remove in the right places. A top hat bush can be used
http://www.kaila.net/tl125/tl125bigbore.html

You must also include the work for trenching the crankases - the NSR already uses trenched cases. Care must be taken to make sure you dont end up with an expensive crank and then break through the cases when further trenching to make it fit - weld and/or epoxy first before machining. See Norbert's 350 big bore page for details
http://homepage.ntlworld.com/ultimatelccrazy/html/casings%20blow%20ups%20.htm

Other options are available such as smaller crank pins and/or narrower big end needle rollers, but it is all a compromise and diminishing returns springs to mind.

If you are after a modest stroke increase, consider an offset crank pin
http://www.mbdevelopments.co.uk/other/gt1.html
This only works if the crank does not have integral big end pins.

Once you have your stroked crank installed in the cases, you need to space the cylinders to achieve the correct port timing and compression ratio and squish you want/need. This can be achieved with custom base spacer plates and copper head gaskets, but some machining will usually be required, especially if different pistons and rods are being used.
Then there is the associated revised inlet work, jetting, ignition and pipes etc...
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NSRless again...