5E - Damage Re-map

I have a number of different products for 5e and they have a different damage types. For instance in a SciFi game I have Radiation and Laser damage. Rather than edit all of the weapons from the different sources I would like to remap the damage types via an index table.

e.g.
radiation -> necrotic
laser -> radiant
...etc

The bit of code I am looking for is the code that calculates resistence, vulnerbility ..etc.

I want to search the damage rolled and if it matches the new data type (e.g. radiation) to replace it with a standard 5e equivelant (in this case necrotic). If it failes to match to let the rolled damage thru unchanged. This would allow any standard 5e monsters to retain some form of resistance/immunity/vulnerbility for weapon types.

It would be good to output the original damage type, but if thats too dificult to do, I would be looking to output the replaced damage type.

Hopefully some one can guide me to the correct bit of code for this....