CLOSE PROX ACR-MLG GA-SMA ABOVE LGA.

Narrative

DEPARTED LGA ENRTE TO CLE ON A HDG OF 310 DEGS ASSIGNED ON A CLRNC TO CLB TO 9000'. WE WERE BEING RADAR VECTORED TO EVENTUALLY INTERCEPT THE SPARTA 252 DEG R ON A 310 DEG HDG CLBING THROUGH 8400' I SPOTTED AN ACFT OFF THE RIGHT FRONT OF OUR ACFT. IT WAS SLIGHTLY HIGHER AND TRANSVERSING FROM THE RIGHT AND GETTING CLOSER. WHEN I INITIALLY SPOTTED THE ACFT I STARTED A DSCNT. AT THAT TIME WE WERE AT 8400' WHEN I PUSHED THE NOSE OF THE ACFT DOWN SIMULTANEOUSLY PULLING BACK ON THE THROTTLES. DURING THE PUSHOVER OUR ACFT REACHED AN ALT OF AROUND 8500' AND DSNDED BACK DOWN TO 8400'. AT THAT TIME THE OTHER ACFT HAD PASSED OFF THE LEFT AND WAS OUT OF SIGHT. WE INFORMED ATC IMMEDIATELY AND THEY SAID THEY HAD NO TFC; BUT SAID THEY HAD ONE INTERMITTENT ECHO THAT HAD DISAPPEARED JUST AS WE MADE OUR INITIAL CALL ON THE NEAR MISS. WE IDENTED THE ACFT AS AN SMA; COLOR WAS TAN AND WAS HDG SSE. IT LOOKED TO BE A FABRIC ACFT. I BELIEVE THAT BECAUSE THE ACFT WAS A FABRIC ACFT THAT ATC DID NOT SEE HIM. TRANSPONDERS WITH MODE C SHOULD BE MANDATORY.