I thought it was the HP fuel pump you took apart for no good reason. Bosch sell rebuild kits of gaskets, bits and o-rings for those. But I think you're saying it was the brake/system vacuum pump, driven by the end of the exhaust cam?
I'd expect any oil leaking from there to be coming from the gasket between the pump and cam box. There's no oil fed to it or pumped by it, just the mist from the cambox itself lubricates the innards enough. Is oil dripping from the joint, or emerging elsewhere?
If it's coming from the joint (the gasket between the pump and cambox), did you replace it? There was a new one in the gasket kit from Autolusso. Are the bolts tightened evenly?
EDIT: actually eBay is a better source for O-rings in very small quantities. I recently got some truly weird sized imperial ones there and managed to fix my 1973 American trolley jack that just started leaking
It won't cause any harm to clean it all up then use a thin film of Granville goop on that gasket and remake the joint. However I wouldn't use it/silicone within the vacuum pump itself without extreme care about where debris might end up. Brakes are useful.
If it's damaged threads giving way, I think you'll need to use Helicoil or similar to insert a new thread. Oversize bolts won't go through the pump body.
I'd imagine the vacuum pump is supposed to be sealed-for-life, but any internal O-rings are likely to be standard sizes, and (if you can measure them accurately) replaceable from the likes of altecweb.com - Products for Home, DIY, Work & Office
In that location, ordinary nitrile is likely to be the material rather than viton (which is hot petrol proof). But I suspect a used replacement pump will not be expensive, and possibly a better idea.