i have had this old sp-1 ar for a long time. now i would like to make it into a flattop but it has the large front pin/screw-bolt. what can i do??

I like the old SP1's.
I would change out the triangular handguard for a round one, thay make them that fit with out changing anything else...

And then I would just buy another AR in the "flat top condition".

Also you can get the ARMS mount that fits the carry handle, mount the optic of your choice, and you may find that the old SP1 is a pretty good AR after all...