There are no mods I know of that can be done easily, but a shop could probably always custom machine different valve sizes to a point, but then you would need custom cams and custom pistons. Not much in the way of that stuff for a 4AFE. It would be a cheaper and more beneficial venture to just swap the engine to something more powerful, like a 4AGZE, 4AGE 20 Valve, or 3SG(T)E.
how much horse power u want
send the head to the machine shop get it shave n shave the valve it will give u more horsepower but it u want more send the complete engine to get dont it about 400 dollar or more but will be getting 100 or more horsepower of of the engine n were n fl u are located
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have the head shaved by about 6 thousandths, the compression will come up slightly. I would not recommend taking off much more. But then again, I am no expert on how much you can safefly take off.. maybe someone else can tell a better figure. You don't want the valves to hit the pistons..
I would say don't even bother trying to get more horsepower unless you want to find a cam, a new carb, and all that stuff... too much money and work for so very little horsepower in return, use the car for what it is, a good transportation car that can be reliable for an old car..