This fizzy is not a "SS" model but more likely a Candy Orange FS1-E.
The frame number is *394-013502* and the non-matching engine is even later at *394-014169" (Pop Purple)
It was not registered until 04/07/1974 so almost a N-Reg.
This coupled with lots of non standard parts fitted has a massive detrimental affect on the value and I would say it is worth no more than £1,000 in my humble opinion!
I have just bought a similar earlier numbered fizzy with matching numbers and original seat etc for around this price.