Any time you're troubleshooting problems like this it's good to check the "easy stuff" first. Pulling out the jets and checking for blockage certainly falls into that category. But, if there was a lack of fuel flow at higher revs it's likely that the mixture would go lean, which could lead to popping, snorting, bucking, and all manner of complaining. You could expect the same symptoms if you had a problem with fuel delivery.
If on the other hand you had a problem with too much fuel at higher rpms, the engine would stumble, and blubber, and miss. That might be caused by a leaking needle valve, or even a float that's set too high (not very likely, that would affect the engine at all rpms). But checking the needle valve and float level is also pretty easy, so give it a look. Too much fuel at speed, blubbering, stumbling, etc could also be caused by something as simple as a clogged air filter. It happens.
A problem with ignition could be a bit more difficult to diagnose. Checking ignition timing is fairly easy if you have a dial back timing light. If you don't, not so much. A weak spark will cause an engine to loose power as the rpms climb, but that's not as easy to troubleshoot unless you just swap out old components for known good ones (like the coil?).
However, cam timing being off a tooth or so isn't always as easy to check. Even if the timing marks line up, you have no way to confirm that the cam is timed "right". If you have all the right pulleys and the correct indicators are installed (it's not impossible to install the incorrect ones), you are probably in pretty good shape. But if it's possible that someone shaved the head, and or, cam carrier, to "improve performance", then even if the factory marks lineup, you might have cam timing that is sub optimal.
An engine will run just fine with the cam timing off by quite a bit. In fact with computer controlled engine management systems it possible to have variable cam timing. Different cam timing for different conditions.
A good rule of thumb is that retarded cam timing will increase low rpm power at the expense of higher rpm performance. Kinda like what your experiencing. Hummmm... Did you say someone else "built" the engine? Taking material off the block, head, and cam carrier tends to cause cam timing to be retarded. Hummmm....
If you have an engine that's been "built" it may be difficult to get the cam timing "optimal". That's because there is no one setting that's best, and, until you degree the cam, you don't know where it is at in the first place. To get the engine to perform as you want it to perform (high rpm performance, low end torque, or whatever) you really need to do some dyno tuning. Definitely not in the "easy" category. A good compromise is to set the cam to "straight up" or no advance, or retard. From there you can experiment with advancing or retarding timing to optimizing the timing. But getting the cam timing straight up, isn't in the easy category either. It's possible, but it's something that many folks are not experienced in.
Just bolting "performance parts" on to an engine can lead to disappointment unless things are tuned properly.
Good luck in your troubleshooting efforts, confirm that the basics are right before you move to the more difficult stuff. You'll be surprised just how well a properly tuned SOHC engine will perform.