First, that's a nifty custom job. Nice.
Second, the Targa is actually another name for the Kingman Spyder Compact. "Rare" is not a manufacturer, it's an adjective. As in fewer guns were sold with the Targa name compared to the Spyder name.
Not sure why they sold them under both names for a while, might have been a distributer things.
Anyways, you're looking for Part #29, velocity adjustment screw...
Bad news is that it's long out of production, and was a very commonly lost part, so it may be hard to find an exact match. Good news is that it's basically a metric allen head cap screw with an o-ring groove on it. The o-ring is just to keep it from vibrating loose, there are other ways to do that.
I think it's a M5 size screw, 16mm long (gun was made in Taiwan, so metric threads all around). Take that bottom plug with you to the hardware store and see what size screws fit.