Today I swapped a coil+solenoid on an ebay 545 runner. To my surprise, the saw had an AT-12 on it... score. Anyway, replaced the seals (one seal I resued with a new spring), replaced the airleaking, torn boot. Saw started easily enough cold following standrad SOP.
Upon running it though, I encountered "revboost" trying to do its thing on a regular non-stuffed crank 545. This caused it to rev poorly from idle. It also just wouldn't low idle. To my layman eye's I figured swapping those parts for something 100% from a 545 (although not same saw) was my next step.
It worked. Kept the AT-12 with a AT7 solenoid.