Hi,
I've been researching wheels/tyres for my own X1/9 project and best tyres for the 13" rims seem to be Kumho V70A details below, as posted to me by a mate of mine at DTweeks:
"215 50 r13 rim size 7"- 8" ......... good stock in all compounds
"235 45 r13 rim size 7.5"- 9" ..... good stock in all compounds.
They're blue book list 1B tyre so are OK in various competition series though not as a production tyre. They're very good though! We use them on the M3s.
Rgds
Mike