So I'm in the process of tearing down a 450 for a friend and doing the usual things that are needed. Cleaned and rebuilt the carb, replacing all the lines and replacing several wires that are no good. Has a broken AV mount and the muffler guard holes are worn out so I got a replacement. Going to give her a good bath and start the reassembly tomorrow night.
I have an old 450 that I like to run. Great old saws, little heavy but when you lay them on a log they'll eat.
Has the muffler been brazed back together yet? These are bad about vibrating the muffler apart.
I'd check the tank for mixing too. These old epoxied tanks seem to be getting old enough that the epoxy bond is letting go or something like that. We have a 450 and a 550 here and had tank problems with both of them. Just finished going through the 550 a month or so back and it was mixing gas and oil so we had to dump some tank sealer in it. I got the 450 a few years ago because someone destroyed the tank trying to split it. I got the saw cheap then bought another scrapper with a good tank. They're getting a little harder to come by.
Good luck with it. Glad we're keeping another 450 in the mix.