As far as heads are concerned, my take:
Performance comes from a combination of many things, but MAINLY from the heads.
I removed my considerably massaged 040 heads: 52cc chamber, 44 x 37, 68cc port volume, 10.2:1 heads with welded and port matched manifolds and had them rebuilt. New guides, a valve job, and one Tig weld on one valve end. Back in business for $500 plus my shipping there, almost $600. Probably the exact same time-frame as new heads.
You can get brand new Panchitos(40 x 35) for $600, add another hundred for port-matched manifolds. If you need the chambers cut it's less than another $100.
I'd buy new, you can't beat the flow numbers and port matched manifolds along with the bang for the bucks. Set the C/R in the low 9:1 range(deck height VERY important) and add those 1.25/1.3 rockers. You'd be less than $1000 into it for heads, rockers, and intake manifolds. The down time is minimal to swap. Don't forget 12mm 3/4" reach spark plugs, all the new heads have dumped the 14mm 1/2" reach threads that strip.
Get yourself an adjustable pushrod(cheap tool) and some rocker stand shims and be prepared to get new pushrods. Set the geometry the right way, there are youtube videos of how-to. You may be able to re-use but you have to mock it up to KNOW.