The cracked plug can probably be welded/repaired by a good metal working 'welder-artist'.
The 8-point (only 6 point
s are triangular, 2 are square) winch handle recess is probably poor design in that its possible to have only 2 of the triangular points in contact with the 'business end' of a winch handle. With only two points engaged it would be very easy to apply too much torque to 'cap' .... crack!
Id be sure that any winch handle is ONLY in full 4-point contact
with this deck fill plug, not just 2 which is quite possible with this design.
O-rings are a dime a dozen from the internet. Measure the OD and ID of the O-ring recess
in the 'housing' measure the apparent diameter of cross section of the O-ring .... all to the closest 0.003-0.005 inches or metric equivalent
Id recommend VITON o-rings, as with so-called bio-fuels, especially those made with 'reclaimed' cooking oil (sometimes added to 'regular' fuels by unscrupulous marketeers), as Neoprene/BunaN will soften, swell and grossly distort in contact with such 'bio-fuels'. You want the softness
of the O-ring in the range of 50-70
Try Amazon; if not, then McMaster-Carr, or a Parker O-ring distributor.
--> multipurpose --> Viton