I am swapping my old faithfull OS 140 for a YS170. I just bolted it in the same mount. I changed the nosering but I didn't try the older one. Spinner ended up in the same position. I am using slightly undersize bolts so I guess they allow for a couple milimiters change.

There is a big change in the CG though. The OS engines itself is about 4 oz lighter and you have at least half of the tuned pipe behind the CG. I was using a heavy 5 subC cells nicd pack to balance the servos (S9650) located in the stab (plane is an Osmose from Argentina) I just put a Thunderpower 2100 Mah 2 cell lipo in place (the regulator was already there for the 5 cell pack) Test flight is today....