I read or heard somewhere that the OS 1.40RX is a drop-in replacement for the YS 1.40. I haven't been able to confirm that.

Anyone know if that is true?

Check the manuals.

Some years ago I had a Synergy where it was originally mounted YS 140 but I swapped to OS 140 RX. If I remember right the was only the nose ring that was slightly different but engine mount was the same. So I think it more or less plugg and play.

Thank you for your response.

The OS would be WAAAY less power than the YS

Not correct at all. I did a direct swap (1.40 FZ to 1.40RX) years ago and he OS delivered FAR more power. And on 15% as oppossed to 30%.
It's a direct swap the only change was the nose ring as mentioned plus of course you'll have to make arrangements and mount a giant 2 stroke muffler and some right thrust tweeks.
I had the FZ definately proppped with a 16x10 and the RX from what I can remember, a 17X12 and 18x10w.

Depends which YS the OP is talking about. The DZ140 is a better performing engine than the 140RX. My DZ140's had an APC 16.5 x 12W.

They are dimensionally very similar but a small weight difference, throttle linkage and the space for the big 2 cycle exhaust. OS was very easy to run but not much on torque or throttle response. Keep a spare rear bearing handy and some OS type F plugs they will serve you well on 15 - 30% nitro fuel. The APC 18.1 X 10 was my favorite