First thing you would need to do with this rifle is to swap the spring (should come with a new one that will bring the fps to where it should be)
Any red dot should work with this once you remove the carry handle
as for a 203 well if you have or get a rail mounted one you will need to add some sort of rails to the front end, once this is done go ahead and add a 203
as for the gun itself I have not had experience with this particular gun