You could write a mod (that has default in depends.txt) that has 'minetest.clear_registered_decorations' then copy and paste all the decoration definitions from default/mapgen.lua, then change what you want.
Adjust the numbers here (except the 1-5 third number):
https://github.com/minetest/minetest_game/blob/master/mods/default/mapgen.lua#L1307