cracks in the exhaust seats

There are two ways it can be repaired. The easiest way is to get an engine reconditioning shop to fit a valve seat insert. This involves machining out the old seat and inserting a new hardened insert. This should be quite satisfactory, and if done properly will be permanent. The other way is to get a specialist welder to build up the seat by fusion welding. This is complicated, and involves stripping the block down, pre-heating, and welding the area with a oxy torch and iron powder application. I wouldn't recommend this unless you have the whole block stripped down for rebuild. It is probably better in the long run than inserts, but is much more difficult. I wouldn't even try arc welding in a critical area like the valve seat, due to the extremes of heat and stress involved in that area. I had the same problem with my Jet some years ago, and the reconditioning shop involved came over to my place and did the job. (One of the advantages of living in smaller community). I did have to remove all the valves first so that they have a clean flat surface to work on, as they have to bolt their boring jig to the block surface to get the alignment correct. I don't know how freindly your machinist is, - you may have to take the car into their shop. Good luck,

Geoff.