No, sadley,,there are no "drop in" per say.
You can buy a blank then breech it, have 3 dovetails cut for sites and under lug, then have the bottom front flat drilled and tapped for the nose cap. then inlet the breech tang and drill taht for the tang screw.
By the time that's all done, you have more into the barrel than what a new kit in .50 cal would cost. CVA doesn't make'm any more but the Traditions co. picked up the contract for the same gun/kit.
Those things always did take a bit of custom fitting,,maybe Traditions offers a .50 rpl barrel that would fit?