I replaced the tube and cam/chain on my '81 about 9 years ago .
The replacement cam/chain part has been changed, the hole where the top goes into, is larger, so now you need to get the 'upgraded' top cover as well or the gear teeth don't mesh at all .
This may be the same thing that's happening with your K series parts .
I checked the K75 part numbers and they are different than the R65 part numbers, so they probably aren't compatible .