Might be a good idea to get it running right before chopping the airbox up? Have you had the carbs off and cleaned them? Checked the float levels? Balanced the carbs? If you have a search on here, there have been other users with the same rizing rev's problem, and these have been some of the solutions. Could be rich because of the float not set right? New plugs I assume? If its got standard jets in it and nothings been touched engine or pipe wise it should caburate pretty well, honda got them pretty well dialled. One symtom of rising revs is an air leak, like you said, the carbs might not be seated correctly, or it could be crank seals, especially if it has sat for a while. I know Dubai is dry and all but still.............
Just my 2 cents worth, but that's where I would start before cutting away and potentially masking (or creating) another problem.