there wasnt much spares or options available for the SR-5 or even the Impress as this was one of Kyosho's kits that just didnt seem to sell as well as they hoped. but to be fair, your particular car is pretty much in mint or near-mint condition. i especially like the "Monaco-Style" rear wing.
i would suggest that you just keep the chassis in an "as-is" condition and just add perhaps a Lotus 107C bodyshell from Tamiya. or if you want, to "keep it in the family" so to speak, look for a Kyosho F-1 body on Ebay.
hope this helps
Racing is in my blood