Michael

For what it's worth, I've never had to work with the stuff you're talking
about.
But we have a few buildings here in Australia with various types of faux
granite, and I've often pondered the concept.  If we are thinking of the
same type of material, then I'd be looking at some of the following
questions:

Mixes - if Mike Edison will do one for you, it's worth considering ... but
equally I'm sure that you could come up with a great mix that would look
damn good and would probably perform much the same.

Patching vs. precasting in the shop - if the original was built out of
precast blocks, or if it is precast molding elements in relative isolation,
then I'd run with that every time.  Better control in all sorts of ways.
But if the original looks like it was poured in large chunks and carved on
site, then I guess you should be trying to go that way.

Finish?  is the finish a rough texture that can be simulated by carving away
at a chunk of patch, or is it smooth such that you need to use a form-liner?

In other words, I wouldn't just be looking at what the stuff is and what it
looks like, but I'd also be thinking about how it was originally
manufactured and installed.

Good luck

david