If the problem is indeed worn valve guides, your only REAL option is to take the head to a machine shop. They will press out the old guides and then press in the new guides. What a lot of folks don't realize is that those new guides will then need to be reamed to the exact size necessary for the valve stem that goes in it. This NOT a job that most of us do-it-yourselfers are equipped to handle. The proper tools are way to expensive for the occasional use we would give them, and it also requires a bit of knowledge and expertise that the average tractor owner does not have.