After an aborted attempt at giving the OEM oil seal another try (it started leaking after only 400 miles of use - see
here), I decided to get more serious about the o-ring option.
Some local searching turned up a bearing and industrial supply house (
Whisler Industrial Supply) that carried a good selection of o-rings.
I bought a few of the following o-rings (they were only $0.35 each):
Buna-N
1/4" ID x 1/2" OD x 1/8" thick
70 durometerI popped one of them into my bike, IAW the OP. I will report if I run into any difficulties. Otherwise, no news is good news....

For our overseas members, since the listed o-ring was a rather tight fit as far as ID went, you MAY want to try an o-ring measuring 7mm ID x 13mm OD x 3mm thick (if such a beast even exists)....just a guess....
UPDATE: Such beasts
do exist! See
here.