Think you need the correct air box and gasket for it to seal up against carb. I normally push the air box hard up against the carb, then snug the 2 screws down.

I have the correct air box if needed.

Yes, the correct air box is absolutely necessary, along with the stupid little expensive sponge seal between the carb and air box. The fuel system on this engine is way "over engineered" to the point of being stupid….So yes, the engine will run lean at high speeds without the correct air box/seal properly installed.
You can just try something to be sure we are on the right track though…When the engine starts to falter at high speeds, simply pull the primer knob out to the middle "enrichment position" to see if this helps solve the problem. If so, the engine definitely is lean…The wrong air box can surely be the culprit, but there are a few other pesky cheesy carb parts that cause trouble as well. The high speed jet is housed in a plastic housing that fits up into the carb nozzle. The jet or brass tube could be partially plugged. There is a flimsy rubber hose that connects from the plastic housing to the carb body as well. These early hoses were known for dissolving depositing rubbery debris everywhere in the carb. Early model hoses were black, the improved hoses were tan in color. There is also a rubber gasket that fits between the bottom of the plastic housing and carb bowl, don’t forget that or the engine will run too rich at high speeds…Does the engine idle OK? If so, the rubber hose is probably OK, the jet could still be dirty though…
In any event, let’s start with the correct air box and seal. Would love to see a picture of whatever someone has rigged up in there now…

I would have thought the 1985 airbox would work fine if it has the sponge foam gasket and is mounted snug up against the carb. The 1985 one is the two part one correct?
BTW i think these are great little motors when the run correctly. I habe an 8hp one that screams and performs way better than every other one I have had