If you get one head, you need to match chamber volume as well as valve size.. Also, you need to determine if it has been ported at all....you will want the flow the same intake and exhaust.

If it is a stock unported head, you should be able to match it.

You are looking at $400 for new heads...minimum.

As far as replacing just the guide, they have up to .010 oversized guides available. You need to determine what size was in there, and whether you will be able to clean it up to get a proper fit with an oversized guide....assuming you can smooth the chamber somewhat.

Really, there is no way to make any kind of accurate determination without hands on inspection.

Perhaps talk to a local cylinder head rebuilder?

You can look for used heads, but if they are in good shape people seem to overvalue them.