My Keeway Dorado sputters & stalls above 40 mph in the afternoon heat.It doesn't do this any other time,.It acts as if there is no fuel,yet if I hold the clutch in the engine revs from plenty of gas

SOUNDS LIKE THE FUEL PUMP OR A LOOSE WIRE AT THE PUMP OR RELAY

1st move is to ck the fuel PSI & emissions. Possible cooling system cause.Scan computer & check sensor readings & the emissions readings are the first moves. Check spark plugs, ignition spark etc. Good luck

PO 720 is an output speed sensor circuit malfunction

That issue will have to be diagnosed & may be the shifting
problem

PO 304 may have been cyl #4 or not & you may have
others going bad as well

You never do just one spark plug

Only proper testing will solve your issues
no one on the phone or internet can diagnose
the actual vehicle,you need to have it

find a real mechanic that can reason out the failure and fix the fault
indicates a problem in the adjustment or a cable problem or if hydraulic , air in the system

find a new service center
indicates a clutch cable adjustment or failing clutch cable

You have a random misfire issue follow the link for info on this problem. Problem 6 Engine Stalls Misfire Code Cylinder Misfire

IF I UNDERSTAND YOU RIGHT YOU ARE HAVING A MISFIRE ISSUE UNDER LOAD MOST COMMON FIX IS PLUG WIRES. CHECK VEH FOR BASIC TUNE UP FUEL FILTER PLUGS WIRES FUEL INJECTION CLEANING

Take the care to the dealer, This car is easily still under warranty and can be diagnosed and repaired for free.