I would seek out a urei specialist, one that works on the studio compressors...
that or find a semi pro with a stellar rep from one of the more advanced DIY type forums....
either way, not going to be quick or easy
I found the guys I use locally by asking around when hanging out with some musicians...
I asked who used which techs, cheap busy tech? expensive, super busy techs? which techs mod extensively, etc? not every one had any kind of input... but those that did tended to rant....
I've been reading up on some older guitar pedal clones and modification...
seems like a lot of info is openly talked, maybe one of those forums super tweaks could help?