Presently, the gear drives are not being made. You might find an unused one for sale, if you ask. An adjustable timing set is available, and this also includes a superior quality chain.
There are a lot of cam suppliers, including the business mentioned above. You could also get your cam reground by a place like Reed Cams. We have found a good few instances of new cams (not regrinds) tending to destroy the distributor gear - food for thought when deciding which way to go, as the superior hardened replacement gear is another cost...
Plenty of headers on the market, in varying prices and qualities. Note however, that all parts for these sixes reflect the basics of supply and demand; being that the market is so much smaller than for a 302 or 350, costs are frequently higher for equivalent items.
Someone will chime in and tell you to buy the "Falcon 6 Perfomance Handbook" which is available from several sources - including the author, who frequents this website. It addresses Ford six cylinder engines such as in your Mustang, detailing modification choices and procedures. You may find it a good running guide to developing your plans further.
To describe a modest rebuild in short form - bore and hone the block if needed, deck to zero, fit hypereutectic pistons, raise the static compression to about 9.1:1 (or higher depending on cam choice), fit hardened exhaust valve seats and do a three angle valve job with minor portwork. Modify the head to fit a Holley 350 or Ford Autolite 2-barrel carb, and fit headers of choice. Include an electronic ignition such as the Duraspark, and recurve to suit your application.
Bear in mind that a radical cam will force changes to the rear end ratio (and torque converter if it's an auto).
Regards, Adam.