I'd go for a 4G63T swap, personally... but that drinks gas like crazy.

I don't think you'll see any major savings in fuel, actually... if that were the point, a piggyback or an SAFC would do you better... but it should feel much peppier at high rpms.

Having never driven the regular 6A12 back-to-back with the MIVEC, I don't know how big the difference in low-end is.