When you say it's giving you problems, what exactly is happening?
If the spring had failed, you would know straight away, with the lever failing to return.
Probably best to start with the easy things; there's an adjuster inside the timing/clutch casing, and the procedure for adjustment is covered in the Handbook on pages 55-57
https://app.box.com/s/hg3dpt09mr1agab0htuj8r35sa7nacztIf not adjusted correctly, gears will be (even more) difficult to engage, and neutral hard to find.
Careful removing the cover to avoid upsetting the valve timing:
"<H2>Removing the timing-side engine cover</H2>
<P>This too was written by the omniscient Mike Whyte-Venables</P>
<P>Remove the clutch cable, and pull the connecter off the oil pressure sensor.
Remove the oil filler/dipstick. Take the points cover off and remove all the
bits, leaving just the bare shaft.</P>
<P>This shaft has a 22m nut that is peened down into a slot in the shaft to lock
it against rotation. Use a very small chisel or narro screwdriver to prise the
dimpled bit up out of the groove.</P>
<P>Take a 22mm EXTENDED socket and a fairly long handle. Give the end a sharp
wack or tug to loosen the nut against the engine compression. The metal disk
behind it may also come out if you use your fingernails carefully.</P>
<P>Pull on the clutch arm to break the seal of the cover. Take a piece of wood,
and tap the end of the points shaft while pulling on the cover. You are
persuading the bearing to slide off the shaft and stay inside the cover. If you
do not do this, the points shaft gear will come out of engagement with the
crankshaft gear, and you will have to retime the engine. This is a good thing to
avoid as it involves much relaxation and at least a Sunday morning to
re-time"
If you're in there to make the adjustment, it makes sense to fit a new return spring at the same time. IIRC, the return spring from the twins fits, and is cheap.
Apparently, the gearboxes have a reputation for being very badly assembled, and benefit from being correctly shimmed. There's an article on that here, although it's not for the faint hearted...
https://app.box.com/s/c9sin2h7gtle3mzem97fjvx21dgxmrm0