When I worked it an engine shop we would mill em out on the head machine and pound in a new one--For the guilds get an expanable reamer,Enco has them and they not to pricey,get one made in USA ,it will last much longer.Then your going to have to grind the new seat cus the guild and seat might not be concentric with each other.All very easy IF you have the right machines.If you take it to a shop have your new parts with you ,it will be cheaper then having them try to hunt them down..
I have seen hack jobs come in with a nut welded on em to try and pull em out and some ground on with a die grinder which now made the hole oversized-----this allways meant the price just went up.
If your going to ream the guilds youself just remenber never turn a reamer backwards,it will dull it real quick and an expanable one will snap like a candy stick.
There might be ways of doing this at home but I don't think you could make it dead nuts on like a shop could.
IMO take it to a shop and it should be as good if not better then new.