Should be no big deal at all.
If I remember correctly, there is an oil seal around the PTO shaft, this could be the culpreit and maybe cost $20 at most.
But there is also a housing that holds that oil seal and bolts on with 4 bolts. This is a gasketed surface. Either a tube of RTV, some gasket paper, or a new gasket. Either way, sould be under $10.
BUT, while it is appart, I would recomend replacing the ENTIRE PTO shaft. Those old ones were 1-1/8 6-spline. Current standard is 1-3/8 6 spline. It is a well worthwile upgrade not to have to ever fool around with adapters again. I think that shaft is in the $150 ballpark.