RE: 90 sized engines
For this size range I really don´t think you can go wrong with a piped OS 91FX, with a Perry pump if necessary. I have been using one in a Desire 60 as a practice Pattern plane, with pipe pressure off a wrap around header. I am about to put in a Perry, to get a better mid range.
There is a trend in 2m F3A, headed by Bob Pastorelo and others (Aerobob on RCU), towards the OS160FX, with a Perry and pipe, which seems to be getting very good results. So the 91FX is a sort of mini version, if you will.
I have been using the OS140FX with great success for tha past 5 years and am trying out the Mintor 170 at the moment, so I am looking to replicate the power curve of these engines on the 91FX, as far as possible.
Sure, the YS 110 is a good motor, but heavier than the OS 91FX which on a good pipe setup gives the same power, at lower cost.