Thermostat Valve isn't a poison (it is there to prevent the engine from slow poisoning) i am using it in my khyber which is currently driven around 2 lac 65 thousand km on it's stock sealed engine i haven't faced any problems with that till date.
These are all just excuses of old engine bla bla bla......
Just all the systems need to be maintained properly.
(Agar sasta kaam karo gy to phir problem to aay gi hi bar bar)