I was able to get the replacement O-ring and it works perfectly! In case anyone else ever needs one for this particular Silca head, I got it at Lowe's plumbing section - had to get a 10-pack for around $2.
5/16" O.D x 3/16" I.D. x 1/16". I tried doing a reverse Challeger-hearing and had the part in hot water for a while but it still split getting it off; it was intact enough to match up.