I agree with the poster who recommended that you tie the anchor up and out of the way while on the mooring. You SHOULD have it ready for deployment while sailing in the event of emergency. You should NOT replace it with an undersized anchor while sailing since the undersized anchor will do nothing to secure the boat in the event of emergency. I wouldn't do anything about replacing the roller. I agree it seems small for the anchor but the anchor seems to sit in there ok and probably rolls off it and back on ok. I would look to see what I could do to keep it from hopping out of the roller in the event of going over some big waves, crashing and banging. You don't want the anchor jumping up and out and then swinging wildly. There are 2 holes at the top forward corner, above the roller. Could these be used to strap the anchor down?
Another thing you could do at the mooring is make a bridle that goes from 1 cleat on the starboard side to the other cleat on the port side of the bow that slips through the pendant. Put the bridle through an old piece of hose cut to size to protect from shafe on either side of the pendant and you will have a good, secure system.