These motors have a history going back to something like 900 CC, so at 1600 and then putting a turbo on it - not so good. They have very little space between their water jacket passeges to the head and the cylinder bore, so they cant take the extra pressure of a turbo. On the other hand the small I4 engines that take a turbo weigh a whole chunk more and they are physically huge compared to these, so these engines really are gems for these cars.
A friend of mine built a lot of these for the oval track guys back in the 70's and 80's. I think you can get 130-140 for street use reasonably well. Duratec engines go to around 175 or so for street. These are all engines around 200-225 lbs.
Trialsman I think you drive this car a good bit so a quiet exhaust is probably good for you. It's right next to your head so your going to hear it enough anyway. Having resistance after the muffler really helps it to it's job, so that elbow you have is a good thing. If you can neck it down a bit there too that's also good. Let the muffler and pipe in front of that restriction average out the flow pulses from the engine, there's plenty of volume there to do that averaging. It won't hurt your power.
Think of those high power superbikes that don't make any noise next to you at the stop light. Many of them have a very small exhaust exit, sometime they just make it look bigger than it really is. They have more power than you and what looks like a 1" pipe exit.
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Wait! I thought pushrods pushed... and rockerarms rocked?