Options
When you meet with your advisers over the meeting with Castro, you come up with three options that the Soviet Union could do to possibly help Cuba in preparation to prevent conflict with America.
The Soviet Union has a massive stockpile of nuclear missiles. You could offer to send some of the missiles, especially the medium range ballistic missiles of which you have the most of, to Cuba. This would likely act as a deterrent to an American invasion, however, it would likely strain tensions between the United States and U.S.S.R.
The Soviet Union could provide financial support to Cuba. With that money, Cuba could use it to build up defenses against an attack. There is low risk to this plan as it would only appear as though one communist nation was supporting another. The downside is that the financial support may not be enough to prevent Cuba from being overrun.
The Soviet Union is a massive country, and that country takes a lot of production to keep it moving. You could throw business opportunities to Cuba, allowing the country to produce goods and build up their GDP. By growing economically, Cuba could become powerful over time. The downside of this plan is that it doesn't provide a solution to the present problem.
