Original MC3's have a Mikuni or DellOrto 12mm carb.
Only the new (since '05) SRs and Runners should have the 17.5mm in stock.
The original exhaust is crapp - Gianelli Reverse runs good on Pia engines. For 70cc i would suggest the Yasuni R. No point in buying only the clutch and the vario, you will get no significant power gain with these parts - firstly go for a good cylinder, carb and an exhaust.