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If you port and polish completely (head, both manifolds), add scavenger exhaust manifold and bigger valves with double springs you might make 25-30%.. (based on Sceptre II to H120 improvements).
Adding modern injection will gain you some power but will probably help more with smooth running & economy. Done wrong it'll run like a bag of hammers in a cement mixer..
If the manifold is heated, ala Sceptre, then setting up an adjustable version will help economy (bypass when weather hot, heat when cold).
I've been using an octane booster, improved average from 38.6 to 42mpg imperial..
Trouble is to do all that won't be cheap especially the EFI conversion (if it's even possible)..
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