Harry are you allowed a different head other than Nissan or stock?

If so a stock Mazda 1300 16v head with 40mm dellortos, 2 exhaust cams,
pipes & megajolt should make nice hp at 10,000rpm if A10 bottom end done right.

Dont know how reliable that figure was but a few of Gordon historic 1000
A10 engines supposedly made 130hp ate with A series heads.
His A10 engines ran prepped rods, Arp rod bolts, full floating forgies with
ladder brace from laser cut 8mm steel and cheaply sourced unbreako main bolts

Mazda heads flow alot more an extra 30-40hp should be possible.