Can I tell, externally, if the head on my B18, Amazon, has been converted to run on unleaded - numbers or summat?

Not unless someone has stamped something on it .. I would just use unleaded with an additive , depending on your type of driving it could be many years before the valves give trouble , and when and if they do , get the modifications done then ...

Keep an eye on your valve clearances - once they're not closing up regularly then all is probably well.

From previous experiance with the B18 and B20 there is no need to have a head converted to unleaded as such. Do a search on my posts about the subject.

I've always run mine on unleaded, never used any additive and she's never missed a beat, bullit proof ;)

Covered almost 2000 mile's on unleaded but use a 4star additive has become a little tappety but told not to worry