Replacing the oil pump is relatively easy on the late 1.5IE models, since there's no distributor shaft to align with.
Yes it can be done underneath, after removing the sump. So it's a relative straightforward job, no gasket
between the pump and the block. On the late 33's removing the oil sump could be a bit tricky, when the engine
is in it's place, because the sump need to be aligned towards the front as far as possible, but the exhaust
manifold (precisely the exhaust pipe of cylinder#2) is in the way. The exhaust manifold is differs from the earlier
(pre '92) 33s.
Regarding the performance problem I've observed the same on late 1.5IE and 1.7IE 33s. IMHO it's ECU related.
Disconnecting the battery terminal for a few minutes will erase the automatic mixture adaptation data, then
engine during the first few complete warmups behaves relatively good at the cold phase. As the ECU finishes
the mixture adaptation, the mixture will be lean(ish) for the cold engine. Both of my 33s are doing the same,
and also at hot engine (coolant temperature above 88*C at RPMs below 2500).