Reagan's foreign policy in his own Presidency consisted mostly of talking loudly and carrying a little stick. His actual military actions, such as Lebanon or Grenada were tiny and tentative. Reagan preferred to hold the coat and shell out cash for other people to do the fighting, or to back covert wars.
I think we'd have seen a much earlier and aggressive involvement in Central America. It's pretty much a helpless region, dependent on American interests, vulnerable to American meddling, with no real rivals.
Between the Panama Canal issue and the Sandinista revolution, I think that Reagan would have taken a much more aggressive approach.
There'd be a lot of political theatre over the Canal Zone. Possibly covert operations to change the government, more likely the right people would be bought off.
We might have seen direct US intervention on behalf of the Somoza family in Nicaragua, or an earlier campaign against the Sandinista.
Bloodbaths is El Salvador, Honduras and Guatemala were already on the way, so I'd expect that timetable to be moved up.
In Latin America, Project Condor gets underway earlier, and the absence of interest in civil rights gives license to a new wave of repression. Latin American Dictators will do very well.
In Africa, I could see Reagan moving things towards a crisis mode, possibly a new Cuban Crises, over Castro's interventions in Angola and Ethiopia. I could, for instance, see Reagan using South Africa as a direct proxy for an intervention in Angola. I don't see Reagan putting American troops in Africa though.
Middle east - I don't see a Reagan Administration managing to handle the Camp David accord between Israel and Egypt. I assume that will be far less productive or effective. Although I'm prepared to believe that Egypt and Israel will reach a rapprochement anyway.
Reagan would be taken by total surprise by the fall of the Shah. American relations with the new Iran would be terrible. We might miss out on the hostage crisis because Reagan pulled the Embassy. Reagan might well overtly back Saddam Hussein, or not.
And Reagan would have no real response to the energy crisis, or the role of the middle eastern states. He'd talk tough, but that's about it.
Relations with China? Hard to say. Nixon opened that door. So its uncertain whether Reagan would keep it open. His position on Taiwan would be a bigger irritant.
The Cold War stays cold.
However, I'd guess that domestic concerns - the energy crises, stagflation, various matters would trump foreign policy considerations. On that front, he'd do worse than Carter.