The
mast sheaves rest on an axle that runs through the mast
head fitting. Only problem with changing sheave is the axle may be corroded in place. Not necessarily true that it will be difficult to remove but it may be. The ones i've done have been a floating 1/2" rod held in place by a cap on one or both sides held on by either tapped screw(s) or pop rivet(s). Remove the fastener holding on the cap and
fish out the axle pin. Easy to get the axle out if it hasn't
corrosion welded itself in place. Personally haven't had any significant problems with the couple I've done. There probably are other ways to secure the sheave axle like running it through the masthead and using cotter pins or a bolt with locknut. In any case it's an easy job once you get the axles loose.
Garhauer is a good affordable source of
replacements.
Harken and probably others make ball bearing sheaves but they are pricey. garhauermarine.com