I don’t think true power hydraulic steering is an option for you any longer as Merten Power Steering sold the power hydraulic steering part of his business to Mercury and they developed their Big Tiller Steering System from it. You may be able to find a dealer that bought some power tiller steering hydraulic units before Merten sold out to Mercury but that may be difficult as most of these dealers are hanging onto these to rig on large tiller boats that will have a motor other than a Mercury on them.
The best option for you with a 90 HP motor is probably the Titan Steering system, which was also developed by Merten. This system is not a full powered system but it has the same principles of the power system in that a hydraulic pump locks the motor in place taking much if not all of the torque away. The Titan has gotten very good reviews on some of the other websites that have had specific discussions regarding steering systems. I would suggest calling Tim Merten at (920) 231-6751 and he can fill you in on all the specifics.
I have the full power version on my 150 tiller and it has been great.