The correct oil for the flywheel is OM13; which has a commercial equivalent of an ISO 10 hydraulic fluid. I the UK, I believe that Morris makes a suitable one. The flywheel isn't too sensitive to fluid type though, and plenty of people run them with regular auto trans fluid without problems (something like a DX3). I'm pretty sure that the seals are available as a generic item.
If your leak is from the bellhousing, I can almost guarantee that it's the fluid flywheel. Because f the design of the housing, they tend to fill up with a pool of fluid, even from a relatively slow leak, that will then be splashed around everywhere as soon as you run it, making it look like a major disaster. It will then settle down to a slow leak again.
I would suggest topping it up with OM13 and possibly adding a bottle of commercial seal conditioner that is rated for automatic transmissions. Then I'd just keep an eye on it to see how much it's leaking. If it's a museum vehicle that doesn't get driven much, like ours, then you might be best to just top it up before use and clean up the mess afterwards, as the amount of work involved in replacing the seals is pretty insane and leaving them sitting around won't do them much good anyway.
Cheers,
Terry