Oil filler cap came off somehow and has led to a little mess which has gone on the leads, distributor and alternator. I was planning on replacing the plugs, leads and dissy cap + rotor arm anyway.

Since the oil has leaked onto the alternator was wondering if a liberal spray of engine degreaser then a hose down would do any damage?

If so is there another way of cleaning it or is it best left as is?

Mop off what you can, then leave the rest unless it's got onto the slip rings.