Re: Need some help diagnosing.
Sounds like it might be overheating when it starts to get that tinny sound and loses power, it may be worthwhile to jet it up a bit richer on the main jet to help with the cooling, or it's time to swap over to a Puch or Fred head.
If the power is just less all around that could just be a result of swapping over to the lower compression head but if it's losing power after warming up and when it starts making that noise, then it's overheating.
best thing to do would be to get yourself a temp gauge so you can monitor the cylinder head temp so you can tell if it's getting too hot, just look for them on ebay or amazon.com, they're really easy to install because the sensor is pretty much like a spark plug washer with a wire coming off it. These are fairly cheap too and they will save your engine since you can always shut down if you see the temp going up too much.