If anyone has attempted to source gearbox parts for an IT250K they would know the rarity of the 3rd/4th pinion gears and the matching gear wheels.
IT250H/J gear sets seem to be more abundant, although the ratios might be different you could retrofit the older g/box in toto to get that K engine built.
The final drive (sprocket combination) gearing is obviously altered simply enough, although there are limitations as to what can be achieved as the results tend to favour one end of the spectrum at the cost of the other.
Whether the power characteristics of the longer stroke engine match the old gear ratios is not known. I would be keen to hear if this experiment has been tested practically, perhaps Nathan could keep us posted if he decides to go this way.