Hi Simon,

Last had this problem around 1980 but I don't remember it being too difficult to sort out.
Source oversize bolt, re tap the gearbox hole to suit. Using the removed bolt as a pattern
drill the center of the new bolt & tap to take oil level bolt. Then source suitable bit of tube
drill out thread end of bolt for interference fit, press fit the tube. measure against the
removed bolt for tube length & trim as required. As for removing the present bolt that can
be a bit tricky due to limited access. Try rotating the bolt while applying downward force
with molegrips. Once finished be sure to remove/flush all the swarf & debris out of the
gearbox. Have fun.

Legion.