...And another little update, I saw a front crankcase cover brand new on Ebay for £20 for the later 24v, so thought I might as well get it so I could compare the old vs new covers, and this well and truly explains where the idea the 116-style sump won't fit on a 24v without drilling new bolt holes comes from:
You can plainly see the bolt holes are in different places, but the rest of the crankcase bolt holes line up just fine with the sump. So to fit a 116-style sump to a late 24v or vice versa, you just need the front cover (excluding the oil pump issues noted above).
And also interestingly putting the new front cover on the old 12v crankcase clearly illustrates how the more modern solid cambelt tensioner is fitted - it does just bolt in place of where the hydraulic detensioner was fitted, just with a longer extension on the front cover to firmly hold it in place:
And finally, some other goodies arrived today
They won't be used quite yet (need to make sure the engine runs reliably as it is first) but the pistons are dramatically different to the originals, all are within 1g of each other, and they're considerably lighter too. But somewhat bizarrely, the gudgeon pins are also only within 1g of each other, and there's one more 'heavy' gudgeon pin than 'light' piston so might have to get one pin machined a little to make them balance perfectly.
Must... Resist... Using... Them!