It is a bit difficult to know where to start.

I guess if that was my bike with those symptoms, the first thing that I would do is pull the exhaust pipes off and have a look up the exhaust ports at the pistons and bores. If the pistons have significant vertical scoring marks, then you have probably suffered a partial seizure. [ possibly due to a faulty thermostat?? or waterpump?? or 2T oil pump?? or fuel mixture too lean for whatever reason?? ]
I would also pull the carbs apart to check that it is running standard jetting. And if not, then think about why not.