Jay,
I believe the answer to your question depends upon the level of performance you wish to attain from the Otari.
I think highly of the Otari for what it is. I own two different models. I purchased them with the idea of cheaping my way into the TP ballpark with a direct output from the playback heads, and a head replacement from Flux magnetics. These are relatively inexpensive upgrades.
Then I started to look into the Bottlehead outboard stuff, but ultimately settled on the Aria.
Unfortunately, you may find that outboarding is the only way to achieve your goals.
When I started to add up all the expense for the performance quality I wanted I found that for me the difference in cost between a 'tricked out' Otari and a refurbished Ampex 102 did not make sense over the expected life of the machine as well as my own life expectancy.
It also had a lot to do with the system that I was putting the RTR into.
Ultimately I did not believe that I would be satisfied with the combination.
But that is me.
I do not own fancy cars, boats, and the like, but I do like my stereo rig and plenty of software.
I am not aware of anyone doing upgrades modifications to stock Otari electronics. That is why I jumped right to outboarding.
Have you contacted bze on Ebay?
He sells a lot of Otari machines and he is quite knowledgeable, and has excellent feed back.
I purchased three machines from him.
You may want to contact some of the repair/service professionals listed in the forum for the real skinny.
I hope you found this helpful.