Hi Bill;
The rubber is a compound and by talking to the right people you can find a
match for what Honda produced. Call Stillman Seal Division XXX-XXX-XXXX,
the area code may have changed since I called them last. But they offered
to do center cam chain rollers. using the original metal centers. Miles is
correct, the plastics don't seem to hold up. I had the rubber dampers for
the sprockets made up with a special compound to bring it up to 80 shore (a
hardness range determined by a Shore Durometer). They do very well and none
have broken up like I have seen the ones Honda made do. Stock compounds can
be used some of which have a Tensile Strength of over 2000psi. But also
look at Polyurethane (millable grade) it may be a way to get away from
casting.
Good Luck
Bill Colford