First, it is very unlikely that your problem is caused by a recent bad repair job.
Valve seats are very hard and replacing them is much easier and cheaper than grinding them much down. If your head struck to the cylinder block it was not recently removed and the person that sold you the car was probably not aware of the worn valve/seat.
Most likely the valve has been to hot (lean mixture) or the valve adjuster failed. The valve then hammers into the head, this may be a very slow process.
Any good machine shop can replace the valve seat, since this is done by warming the entire head it is best to replace all seats that look worn. After that you will have to grind in the valves. If you have no experience it may be advisable to have the shop do it.
Also replace the chain, tensioner and guides, it will be disappointing if you have to do a head job again.
You can not expect to buy a 83 car below market value without some wear. If you feel this is a ghastly problem you may be better of with a new car. Old cars can be fun if you have a relaxed view towards mechanical problems.
Hammering on the valve to release carbon deposits sounds like a strange advice. The carbon deposit ring on an old valve helps to seal the valve. If you remove this carbon you will have to grind the valve to make it seal again.

Rob