Broken reeds can cause hard starting, but I doubt this is your problem, try the simple stuff first. You have to pull the barrel to get to the reed valve on those early RMs. Believe they were steel reeds which don’t tend to crack so often. Course they could have been replaced with aftermarket, however I wouldn’t expect it to start so easily after it was warmed. Plug Caps fail all the time.
Check that fuel tap (assume you DO turn it off every night yes?). Weak ignition could be a bad earth, the ignition coil (under the tank), maybe the CDI, or the stator coils (flywheel coils). They are known to have a limited lifespan & old RMs are weak sparkers at kickover speed. However weak stator coils would tend to get worse with heat & possibly break down at high rpm. Try the simple stuff first, including yet another plug, but disassemble & blow through the choke cctry do not poke wire through anything.
My 2000 model GasGas has spark so weak at kick over you would swear it has a problem & will not start from cold until you learn how to turn the choke on fully, then it starts 2nd kick.