Ok, so it's an 80Hp version of mine!
Impeller change is easy. 5 bolts round the bottom of the leg, one hidden under the trim tab. (remove the trim tab by the bolt accessted through the grommet on the top above, if it's still there! Only thing to remember is engage forward gear before you drop it - the gearchange is a rotating rod rather than the more conventional "up down" motion, and the gearbox will spring into forwards if the change rod isn't holding it elsewhere by engaging forward at least the shift mechanism is in the right place when you reassemble. When reassembling, just make sure the gearchange splines are engaged before you tighten the bolts. the drive shaft will have a spring loaded pin on the top end, so no faffing about with packing shims etc.
The only thing that the 50/60 clamshells were reknown for was destroing their top piston. Hopefully Uncle Al or or TurboD will be along soon to confirm or rubbish this, but IF the 80 is a similar design, on mine the top carb has a slightly bigger (+0.002") jet to prevent the top cyl running too lean & hot. THere was also a small reduction in spark advance & max RPM to prevent those failures, but this may not apply to the 80, and if Steve's engine is running then it's either not a problem on the 80 or his has already been modified -there is no "halfway" with that failure - it either runs or it's f*cked".
As for fuel economy, they are lightweight engines for their HP, which seems to counter the inefficiencies of an old design. As I have said above, I get 0.83L/NM out of mine on a 1/2 ton deep V hull. IMHO at WOT 80Hp needs a certain amount of fuel to generate the gee gees, and so the economy at Vmax is going to be on a par with most 80s. If you spend all day pottering at idle, be parpared to cry.... I did a canal trip in mine to the Falkirk wheel 10th anniversary. Spent 7 hrs between idle & 1500 rpm doing pretty much walking pace & got 2L/N Mile out of it!