The 90 fan shrouds and clutch aren't essential to have ,
but good to have
,especially the fan and shrouds.
The fan and shroud shifts more air for a lower rpm than the smaller diameter setup (or one would hope it does,seems to me to have more draft than the 60 size did anyway).The 90 clutch is weakened slightly on the wall area which i think is intended to make it bite harder at a lower rpm.The one i got did not seem any heavier in weight.It seems to work ok with the 60 size clutch as well,it did for me anyway
The frames have to be slotted as the clutch bell has to move further from the maingear now as the 90 size pinions (15,16Tooth)have more teeth and increases in their overall diameter.Because the pinion block and clutch bell has to be moved the engine also needs packing pieces under the engine beams the same distance/thickness.
One of the big problems the Millenum has with the 90 engines (90's accelerate it greatly)
is it rags the pinion's bearing contact surface something terrible and polishes the inner surface of the pinion support bearing making it a very slack fit and enventually can cause glitch hits if not attended to.
It seems to do this even when locktighted and lined up to the n'th degree with an initial tight fit
If you have to buy a new pinion for 15T or 16T for this conversion
i would go for the double bearing support Millenium 3 version.
You may need a different upper block too
I think they only do the double height one in 16T but i may be wrong
Another thing which should help is the new sideframe stiffener pieces,
that Russ Deakin developed .If you have some spare cash,all the 6 alloy frame spacers from Quick UK,or the full set including the through braces from Ramster would be good to have on it too with a 90
Rc-Market does a full CNC support cage for the pinion too but i think it may break clutches unless you use the frame stiffeners from NHP/Revolution Models to reduce the frame flex
I have used the MPII with the YS91 but not in the Millenium,with a Futura SE.I've only used the older OS91 and the C-spec in my Millenium.The Hatori SB's are supposed to be good with the YS91's too.You could try a Zimmerman muffler but with my OS91's the Muscle PipeII pisses on it for power with 10% nitro but isn't as smooth running