+1 to what Steve H said. At some point, you will most likely have to replace the valves, guides, and seats in that bike, like all airheads prior to about 1986. Best option seems to be to go with new BMW parts - there are other aftermarket items out there, but general consensus is that the BMW ones may be slightly better in some ways. Then it will be good for 100K miles on top of whatever it already accumulated.
The valve stems on the R45/65 bikes were 7mm as opposed to 8mm on the R80/100. The R45/65 bikes also rev more quickly and are happier at higher revs than the R80/100, and due to their low gearing, need to rev more. These factors do put a bit more stress on the valve train to some extent, but the smaller bikes also produce less heat than the larger bikes. Unless it was severely abused, your R65 isn't a 2-wheeled moto-grenade. Just get her all tuned up and ride her, and just keep an eye on the valve clearance. If it starts closing up again rapidly (say .002" or more in under 2000 miles) then it is time to send the heads out for work. Anticipate ~ $500 for the cost of parts and labor at today's rates.