Found budget 1.6 +1.25mm oversized exhaust valves
 

So I thought I would share what I found for the internet's collective knowledge but I know it is a 1.6 so no one cares...

I was killing some time looking at a valve catalog and found that the '96-01 Hyundai Elantra 2.0L exhaust valves should nearly drop into a 1.6/B6 head. RockAuto has the Enginetech EM3467 valve for $6.28/ea or $50.24 for the set of 8.

The OAL is 0.0035"/.09mm shorter but you can easily sink the valve while re-cutting the seat to solve that. The only "issue" comes from the stem diameter, as it is 0.001"/.0279mm thinner; however, the matching Elantra exhaust valve guides (SBI 1408571 -$24 for 8) are spec'd to the same OD with a slightly tighter ID compared to the Miata guides. The guide's OAL is 0.07"/1.778mm longer and will need to be trimmed for the proper guide installed height.

So for $50-$75 you too can upgrade to +1.25mm exhaust valves for your screaming 1.6 turbo build and save almost $100 over the supertech SS +1mm's Or end up buying a set of NOS SBI Inconel std size valves for $80 on ebay like I did. I killed time hooray!

Bonus oversized intake valve info: The BMW '91-00 328 intake valves are a +2mm (possible candidate to backcut and profile down to +1.5mm/+1mm) with a good stem diameter. The obstacle on this valve is that the stem is 0.0514"/1.306mm too long and has 3 keeper grooves. Rockauto - SBI 10695 for $7.12/ea. I'm tempted to grab a few and see how the BMW keepers interface with the Miata retainers OR what it looks like welding up the keeper grooves and re-cutting the stem tip for a miata 1G keeper followed by heat-treating/case-hardening or fitting lash caps but since I would have to cut an oversized valve seat first, I need to find a "junk/free" 1.6 head since I'll be staying stock for now.

Cat awarded

The reason the aftermarket manufacturers offer 1mm over valves is to offset the reduced installed height from cutting the seats. Sinking a valve always reduces output.

I agree on not sinking valves if you don't need to. Since the stem length is only 0.0035"/.09mm shorter the valve installed height may well be within the FSM range anyways at original seat depth. If not sink em a smidge (easy/cheap/worse) or cut the spring bases down(better/harder/more$).

Could just use a thicker shim.