Its probably the KM type gearbox (as I'm pretty sure it has a cone clutch not a plate type). I've just looked in the manual and there is a choice of three ratios. I have no idea which, and as as it is 2000 miles away will struggle to find out..Snookered.
Not easy from 2000 miles away - but is easy if you can see the top of the gearbox as it is on a plate. Essential to know as it makes a massive difference to the diameter of prop you can use. Measuring the existing prop will give you a guide, then you can do the calcs based on each ratio. I expect it will be either a 2 or 2.6 as don't think there is room to swing a big enough prop to use a 3:1