Sounds like it wants flushing. There are several products available for car radiators such as Holts. I had a BSA Three wheeler and that used to boil till it was flushed out. The water ways on my Scott are only about half a mm or less and I was surprised how much grit came out of the spare just by shaking.
If your engine still overheats , then the radiator might be being asked to do too much and the mixture strength might be a bit weak. Heat transfer also depends on ambient , so warmer days will affect it. All my other two strokes are air cooled and two do have an under the plug bimetal heat sensor so head temperatures can be monitored. Quite interesting when the reading gets high!!!!
Another point to watch out for is trapped air in the shoulder of the radiator that is the other side to the filler neck. Gases expand far more than liquids as they heat up and so displace the coolant out of the overflow. Always ‘burp’ that air out by leaning the bike well over so that the trapped air escapes over to the filler neck side. Perhaps a two-man job if it is a heavy bike ! Don’t top up coolant to the level of the overflow pipe as you will only lose coolant as it expands. Keep it a 1/4″ or so lower.