365xt is the same bottom end as the old 372xp so everything major fits
I know for a fact that you can buy a OEM 75cc XPW top end the 51.4mm piston and cylinder and they will bolt right on the saw or the 50mm either combo will fit . I am pretty sure Treeslingr has 2 50mm P&C for sale in the classifieds for a reasonable price Like $135 OEM that cheap. But anyway you can turn that saw in to any version of the 372xp that was made even the big bore P&C will fit get one from MCW on here just PM him and ask for a 372 BB kit leave the base gasket out, put Caber rings on the piston and throw the rings and C-Clips that come with any of the BB kits away those are the weak links the rings from a 460 Stihl will work They are 52mm x 1.2mm,and use the OEM C-clips. But keep in mind if you change over to the old style P&C you have to change to the old style Carb and intake tube. The guy on you tube named " The Chainsaw Guy " sells the complete OEM BB change over as a kit that's probably the way to go or buy one of the P&C off of Treeslingr and get Simonizer to port it for you won't be disappointed he is worth every penny.