I knocked off the valve with a sledge then filled with water and drain. You will not have an explosion after that. You can then cut and weld on it all you want.
I did something similar. I used a hydraulic motor and chain/bar. This is run to my aux hydraulics. Works good for small jobs. Could use it longer at a time but I don't have a chain oiler on it and have to oil it after a few cuts. But works great.
I had exact same issue on an older 6' Bush Hog myself. I ground it out and weld it up. That was 3 years ago and still going strong. Balance wasn't an issue