Valve seats are made of very hard metal and are pressed into the head. There are different quality seats. In my cars, since running on GPL, I always used extra hard seats. Normal seats are less durable for engines running on GLP. But even the “softest” seats are much to hard to ream easily, you will need a special machine and very hard (expensive) chisels.
I would take some time to find a machine shop with the necessary expertise, maybe the dealer can help you finding a shop. I live in the Netherlands, all machine shops over here have experience with European engines. But even over here some shops are much better than others. Some shops go for the quick repair/low price, others for high quality. The shop I go to also repairs vintage cars. Last time I was there the were working on a pre-war Rolls engine, that was a nice engine!!

One of the problems with old cars is that over the years a lot of people may have worked on them. Repairs, especially before a sale are not always very good. You just cannot expect the previous owner to have the engine repaired at a price that may exceed the estimated resale value.