Here's a link to a video I just made tonight on the Miata. I had a long-distance person interested in it, so I made the video to show the car. Unfortunately as you hear at the end of the drive, the banging gets worse. It's officially D. E. D. dead now. It'll start and run, but the stupid thing won't even go over 50MPH.
Any ideas? Notice the new factory plug wires replacing the Magnecors, all the new crap under the hood, and the fresh oil coming out of the exhaust cleverly disguised as smoke.
Is it getting fuel and spark to all cylinders?
Check to make sure that it hasnt stripped a spark plug out of the head. I have had it happen 2x with different heads.
check the throttle cable 
Leak down time :-(
Just checked the plugs when I put in the new wires, they are all in and tight. I used to own a 4.6 Ford, I know what blown spark plugs are like 
It is getting spark to all 4 cylinders according to the timing light pickup. I did not check fuel though, maybe test light poked into the wiring at the injector?
Throttle cable? Um, it works? It goes WOT, it just only makes 3HP and sounds like someone stock a rock in cylinder #1, a Hot Wheels in cylinder #2, and a very angry gerbil under the valve cover.
I also did a compression test, it's good on all 4. I need to make a rig up to do a leakdown test though...
sounds like bad lifters/followers. My saab made that noise before I bled the air out of the head's oil passages.. and it had no power either
"before I bled the air out of the head's oil passages.." What? What? Is not an oil passage self bleeding?
Sounds like the Miata's jumped timing.
What's the oil pressure like?
saabs are known for trapping air in the head's oil passages after you remove the head..
I ran this by Bill at FM, and he pointed out your oil pressure gauge never actually moves. That's not good. His vote is for a collapsed lifter, I'm thinking it's lower down. His quote was "I'd be shopping for a 1.8" 
Miatas aren't known for trapping air in the head. I'm not going to say it can't happen, but I haven't heard of it.
Bad lifters is the most likely cause. Pretty sure Miata's don't get air in the oil though.
Timing was spot on, just checked it right after the video.
The wire broke on the oil pressure sender right before the video, must have snagged it getting at the filter. Before that it would jump right to 90, settle to 60, and then down to 30 when warm. Pressure would increase with RPM's as expected. I should fix the wire though...
Keith wrote: His quote was "I'd be shopping for a 1.8"
I've learned that Bill dispenses very good advice.
I've seen 1.8 long blocks change hands for $300. Not every day, but they are out there.
Cheapest I've found locally so far is $550, but that is delivered and with a 90 day warranty.
1.8 time.
Shot lifter(s) and/or jumped timing.
Even with one shot lifter, i would think it would be capable of over 50mph.
Unless one of the cams jumped a tooth, it should be right on, though I guess I could check that and do a leak down test. Looks like I need to work on it this weekend.
I don't think I've ever seen a Miata jump timing. I have seen them mis-timed (that intake cam really wants to be one tooth over) but they don't make that noise.
I'd start with fixing the oil pressure gauge. It's only one wire, easy enough to do and it may tell you something useful.
I never looked hard at a miata engine.. but I assume timing chains?
This is a 1.6? How is your crank nose?
They use a belt. The crank nose failures don't usually make noise like this, they just act like the timing is retarded.
timing belt... hmm.. could be a jumped tooth or two. How hard is it to start?
it's been twenty years since I had a jumped tooth in a fiat (actually closer to 15) but I remember it made a LOT of noise with only one valve slightly bent.
What Keith said, it's a belt. It's also non-interference though so there will be no bent valves. Also, I completely redid everything on the crank snout of this car already (woodruff key, crank bolt, crank hub, harmonic balancer/pulley, all new Mazda parts).
I'm going to fix the oil pressure gauge wire, and put the motor at TDC and check the cams to see if they still line up. Hopefully this weekend if I finish enough Honey-Do's. 
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