I have 7 Hydro 90s that I run. I had all of them rebuilt by Brian Charney. The main problem with this motor was the front bearing was too small and prone to failure. I bought a huge lot of parts here on IW, the lot came with many modified bearing housings that all addressed the problem by allowing for a larger front bearing. What does Brian due to the motor, he will strip and rechrome the sleeve with hard chrome, it will be hand honed to fit your piston, you will get incredible compression and much longer life against wear, Brian has made steel bearing housings (I purchased one but it very pricey) the motor timed, some porting, custom head and head button. He will chrome the crank pin for extra charge. Drum rotor timed, if is a disk motor it will not last as long. Its not cheap but his engines have distinct sounds, throttle well and are explosive at launch.
My advice, run you motor but I would not put a lot of time on it between bearing changes, do it more than usual. Parts are out there but the motors have been out of production for a long time so good or new parts are going to be hard to find.
Good Luck, its really pretty all polished up.