How do I put a 220V motor and compressor on a 3 phase air compressor?

If the tank is in good shape (not rusty inside) I would get a new motor. You can buy them at www.northerntool.com and even www.harborfreight.com sells them.

If I could get my hands on a large tank for cheap I would do it in a heartbeat. Just make sure you get a motor that does not exceed your power in your shop.

If you have an account or know somebody that does you can get compressor motors at www.grainger.com

you have answers that assume you are using the gas engine to generate electric power to operate the 3 phase electric motor that runs the compressor. That means you need a 4 kW alternator on the engine with the proper voltage and frequency, not very likely or easy to do. I think you want to couple the gas engine directly to the compressor, but then why are you leaving the electric motor in place? But anyway, 3 kW is 4 HP, so you certainly have plenty of power. But you will need to provide gearing or a belt drive to get the compressor turning at the proper speed. .

All you need to change is the electric motor. But you have to get one with a horsepower rating that is big enough to drive the compressor. You may have to upgrade your house wiring to your compressor beacuase it'll probably be a large electric motor that will draw a lot of amps.