Absolute number one priority is change the oil as often as you can afford especially for MX or Road trail four stokes. Every 4 - 8 hours ride time would not be excessive and if is a race bike then every or at least every second race meet should be full of brand new oil.
this will increase engine life and decrease engine wear an enormous amount!
Ps change the oil filter/s every 2nd or 3rd oil change
with the oil Your clutch rely on it - Your cams rely on it - Your transmission rely on it - your crank rely on it
2nd priority is clean air filter if your mainly road use then not so often but if your off road then same as oil. Change as often or clean as often as possible
The rest such as head gaskets,piston / rings, valves and clearances, spark plugs, clutch plates/springs - Really no way to tell all depends how hard you ride just address or investigate when power loss or hard starting occurs
But I guaranty all the above will last heaps longer on clean oil and air