Stalling after 15 to 30 minutes of hard riding.

There are many possibilities for this issue here is a list of things to diagnose first of all ... when we get the reason we can then talk on how to rectify...
Check valve clearances are they in spec..
do leak down test to check for leaking intake or exhaust valve this also tests the health of your piston and rings by default..
With a multi meter test the resistance readings on your stator you will need your manual or ring Honda to get the range of in spec readings on the stator resistance..
one of these tests above will definitely give you an answer and let you know what the issue is -
if all the tests above conclude that the bike is all good then the CDI or spark plug coil is the only area left.
In my experience a bike that starts, runs good then fails, is nearly always electrical (stator low on resistance) but if you do all the tests above you will get valuable information on the engine and the electrical internal health,which is always good to know !
good luck mate regards Jamie