I don't know if Policarpo Lopez was involved, but if Castro and/or Cuban intelligence encouraged Oswald to kill JFK, would that constitute their "being involved in the JFK assassination"?
That would make them accessories before the fact and would mean Oswald was part of a conspiracy. If I encourage John Doe to rob a particular bank and he robs that bank, I could be charged with conspiring with John Doe to rob the bank as an accessory before the fact.
Your OP suggests a more direct involvement by Lopez. You say,
Was Policarpo Lopez the "Oswald" that Deputy Sheriff Roger Craig claimed to hear whistle loudly and watch run down the slope to Elm Street and hop into a Rambler station wagon driven by a dark-complected man about ten minutes after the assassination?
Lopez's pre-assassination and post-assassination activities are suspicious, if your account of them is accurate. We know the KGB was aware of, or suspected there was, a plot to kill JFK. It is plausible that they enlisted Cuban intelligence to look into/track/discover the plot. This does not necessarily mean the KGB or Cuban intelligence were behind the plot.
The problem with the Castro-did-it theory is that Castro was actively pursuing better relations with JFK and the U.S. If anything, Castro would have been motivated to discover and prevent any plot against JFK because he knew that the more-hawkish LBJ would become president upon JFK's death.