MY opinion? Upgrade to billet parts, then upgrade valve springs.
But really- this IS about opinion, isn't it. Hence, my previous point.
Sorry what I meant was when you upgrade valve springs, upgrade to billet parts. Billet parts are not neccesary with clone 10.8lb or less valve springs. Sure, billet parts and valve springs should be one upgrade, done at the same time. JMO.
How easy to install parts? With a torque wrench and correct sockets, not too hard.
Best way to limit RPM is with your foot, or a governor. Rev limiting ignition systems are expensive and I am not sure if they are even available for the clone.
You can get a tach from china for $10 on eBay, search 'tachometer hour meter'.
If you limit the rpm with a throttle stop this will not be consistent. Under load the more requires more throttle for the same rpm. That's how a governor is good.
If you wanted to keep below 4500rpm, I would reccomend leaving the governor in and adjusting spring tensions and positions. Use the cheap tach to get an approximate figure.
Floating valve springs are a pretty good limiter too, but the valves and retainers will wear quicker, and you are more likely to drop a valve.