Glad you have decided to take it down. It's really not that daunting unless one is very uncomfortable with a wrench in his hand. Biggest challenge is usually the flywheel Gibb keys. If a suitable puller is used FIRST (instead of after the heads are messed up by bodge methods) they will come free.
The oil pump's role in a CS type can easily be misunderstood. None of the CS types are pressure lubed even when they have an oil pump. The oil pump in a CS or clone serves two purposes, it supplies oil from the lower sump to the bushing type mains after which the oil runs into the upper shelf in the sump where the dipper on the connecting rod picks it up and splash lubes the rest of the internal workings. The excess oil in the upper sump overflows into the lower sump. Thus in the original design the lower sump can virtually empty yet the engine will still be fully lubricated as long as there is enough oil to keep the upper sump near full.
Several things for you to consider prior to going to all the effort and cost of adding an oil pump, this is assuming your engine is similar to the handful of Metro engines I have had my fingers inside of.
A- Your Metro has tapered roller main bearings not bushings and they are happy with what ever oil gets splashed on them.
B- The Metro does not have the second shelf in the crankcase thus lubrication is dependent on proper oil levels being maintained.
C- Go on with your build without an oil pump. When you get it running again start it up and let it run for a minute or two. Shut it down and quickly remove the crankcase door (warning have plenty of rags handy) take a look inside and decide if any additional oiling is needed,