GasGas MC 250's normally come with the twin chamber Marzocchi 50mm forks which are very stout compared to the 43mm WP which are normally found on 2002 XC300.
The 50mm Marz. are great motocross forks but be aware that stock they are going to be a little stiff for roots and rocks. Revalving (forks and shock) would make a big difference for making the suspension more off-road compliant. We've done this mod before and the even though it is a little pricey the 50mm forks set up properly are awe inspiring both off-road and on the track.
Normally MC 250s are sprung a little heavier up front with a spring rate of .44. The rear shock should be at 5.4. Both of these spring rates are stiffer than the normal stock springs of your 2002 XC which would have been .42 and 5.2 when it left the factory. At your weight (240 - 265) the stiffer MC spring rates would most likely be better for you but again I would recommend revalving both ends.
As far as the heavier flywheel weight goes the MC 250 probably has the same ignition as your XC300 (while looking at the ignition cover on the left side, the ignition wires would leave at the 10 o'clock position) so any additional weight would help smooth out the power delivery of the MC 250 as normally they are quicker revving than even a stock EC250.
It would also be to your benefit to find a 2005* or newer MC250 GasGas that has the dual mapped "Hi-Lo" ignition. *( SOME 2005 MODELS HAVE THE DUAL MAP IGNITION BUT ARE ONLY MISSING THE "HI-LO" SWITCH. 2006 AND NEWER GASGAS BIKES SHOULD HAVE THE HI-LO IGNITION)
To smooth out the MC250s power delivery even more I would recommend a RB Designs head and carb modification and/or a G2 "cammed" throttle set up with the least aggressive cam.
Off the top of my head the suspension work will run around $600 to $700. RB head mod is around $60 and the carb work is around $160. A G2 cammed throttle set up is around $90.