Hi Rebel moby, thanks for your advice, I've learnt a huge amount by reading through your posts about moby tuning.
Very good point re plug shrouding, I'll open it out a bit, there's plenty of material there to work with.
I bought the barrel amidst late night whisky fueled enthusiasm, when it arrived I found I'd ordered the wrong shaped one. There is nothing like treats here so I'm ordering bits from france, returning it was too much of a ball ache. I then started researching what an av89 engine was and found the hc head, buying a new decomp was going to be £16 but the hc head (inc decomp) from france was £24.
Not sure about gearing for speed just yet, wisps are renowned for their terrible handling, I measured the trail at a 8mm!
I am very wary of removing too much material from the cases, not rushing it, I'll definitely do your oil feed mod. Any and all advise greatfully received.