The injection pump, intake manifold, air filter, and exhaust (and even the ignition) on your '81 are all different from the '72 -'74 Spiders, which is where you'd want to source your parts to get the best bag for your buck. I wouldn't bother just changing cams, or exhaust on an '81, as the improvements will be minimal - the restriction on the intake side will still be there. Your best bet it to find someone who's removed all those parts from their earlier Spider and installed carbs and headers in their place, hoping for a performance gain. Whether or not anything runs better than an early Spica Spider in optimum tune is a matter I'll leave open for others to debate, but my '74 is intact, and it's staying that way.