As Mahoney says, no problem in changing the cover but there are 2 types of gasket : Early and Late.
The gasket only seals the water-ways though so if there is oil leaking its probably coming from higher up on the head, like the cam cover gasket or the half-moon seal.
Hmm. It's not easy to see back there. All I know is that there were oil drops on the starter motor and i assumed it was coming from that plate. Will try to have a better look with a mirror.
Although honestly that's the least of the oil leaks. I rebuilt this engine and gearbox 18 months ago with all new gaskets and oil seals, but it makes a real mess on the drive, mostly from the bell housing. I think it is the back of the engine that is the culprit, but it could be the front of the gearbox. Whatever, the only way to fix it is to remove the lump again. Joy.
It will have to wait. The car is booked in to the Silverstone Classic weekend at the end of the month with the TSSC. 380 miles each way should test it out properly!
There is a known problem with replacement rear main oil seals on slant engines, insofar as the wire ring doesn't have sufficient tension. There are a couple of potential cures on here by Jeroen and JT. Or get one from a known supplier of decent ones, I believe Robsport is well thought of in this instance.
The gearbox front seal is a less likely culprit unless a) it's an early 3 rail box, which shouldn't be in your car and b) you habitually park it nose down on a steep slope!
I know this doesn't help much when you are STILL facing an engine out situation, please accept my sympathy, but not too much, YOU bought a Dolomite!