Simon's right, smoke on startup is usually the valve seals.  The 410
engine is a B+**$% for smoking . . . Whilst you are at it give the
piston rings a thorough Redex treatment (more smoke only white this
time!) to make sure they are not gumming up.  Blow by is a common cause
of people giving up on their 410 when all it needs is a good cleaning
out and de-gumming!

Run engine 'til hot.  Take out one plug and turn engine until piston is
just a little down past top dead centre (TDC), squirt in a three or
four teaspoonful dose of Redex, screw plug back in and leave to soak
for a few hours or overnight.  When soaked, take out plug, put a rag
loosely over the plug hole and crank the engine to expel any liquid.
Replace plug and fire up . . . lots of white smoke!  Do each cylinder
in turn.

This will often resurrect a poorly engine and get rid of oil blowback
from crankcase pressure.  It's worth reaming out the pipes from the
rocker cover to the airbox too.

Hope that helps!  Lurch