First a disclaimer: I am an engineer
and not marine
or structural so any opinions should be checked carefully with a qualified marine engineer
as this system is holding up your mast and failure could result in major damage or injury on your boat.
In my boat the base plate was held in by four lag screws. The keel design is lead encapsulated in fiberglass
so the lag screws holding the mast step assembly go right into the lead, no concrete to deal with.
In your case it sounds like the step is over the concrete so you have to deal with some sort of device to hold the lag screws. First you should understand that the load on the mast step is mainly a compression
load, straight down to the keel. Yes there is certainly a lateral component that will push the step fore and aft and to the side but much less than the compression
load. So the requirement for the strength of the screws holding the step to the keel is not that great.
So I think using lead, expandable anchors in the concrete keel would work. Are you able to probe into the old screw holes with a tool to feel the concrete around the hole to see if it feels fractured or pulverized? That would be one of the issues I would want to check.