I think you are on the right track, what you have sketched up would absolutely work. I am not sure how long it would run a 7500 watt generator, I would guess it would probably run an hour or two though, depending on load? Maybe someone else will chime in with better estimates. The size of the engine is pretty flexible with charcoal, but a bigger engine obviously will burn through the fuel faster, and also make a bigger hotter reaction that will overheat sooner. Bear in mind that you will not get the full rated gasoline output - so if you need 7500 W in output, you will probably need the generator to be rated a bit higher.
The only change I would suggest is the nozzle. The thread on charcoal nozzles is worth a read, and Steve Bowman made a nice example of the flute nozzle using threaded iron pipe.
You probably would not need more than one hole for a small engine. This style of nozzle has a good track record for durability. Looking forward to seeing it up and running!