Replacing Thermostat on Winterwarm Small Insert

[Green Mountain Man]

I rebuilt my Vermont Casting Winterwarm small insert last summer. Took some doing gathering parts from many different sources (not one, or even two, or even three, had all I needed... refractory unit was the most expensive, but needed to replace some cast iron parts that had cracked, likely from over firing sometimes over the years?). Things are working beautifully now, burn seems clean. One thing though is that I don't feel the usual resistance or hear the kind of clink of things closing when I operate the thermostat control arm (or it's variable). Wondering if I should try to replace this... and wondering how difficult it is. Do I have to snake a cable deep into the heart (back inside) of the unit? I may just wait until the end of winter to take this on. Bolts may be frozen and want to break removing the thermostat cover.