I just changed out my oil gauge from my 83 300D 2 weeks ago. Link below. It is much easy if to replace the section with the oil, temp and tank level gauges with one from JY. I ended up replacing the oil pressure gauge only, it is very fiddly but can be done. You need small plyers and a small angled screw driver.
I also took a complete section from a JY. It is a 79 300D model and it is a direct fit. I did not use it at the end as it has an aluminum circular cover in the middle which I do not like. I will sell it dirt cheap if you want it. Just PM me.
Engine oil under the dash 83 300D
ODO can be fixed. There is a lot of write up here. Clock can also be fixed. Just replace the 2 big capacitors on the PCB circuit board. Link below with a lot of MBZ repairs.
http://articles.mbz.org/instrument/renew/