You shouldn't have a problem with an OS 70 in a Raptor 90SE,
but you should change the gearing.
(A Raptor 90 is, mostly, a Raptor 60 with a longer tail boom, and longer blades.)
Check your engine. I run my OS70 one turn out on each needle.
Check your throttle settings, make sure the linkage is RIGHT.
ATV/EPA 100/100, Right angles on servo horns, 50% throw
on servo = throttle half open.
If you want to keep that engine in the 90, you'll have to 'downgrade' some parts to the 60, mainly:
Switch the gearing -- the Raptor 90SE gears are designed for
a 90 size engine. The OS 70 is designed for 60 size gearing.
Try a 10 tooth pinion and 93 tooth main gear, the gearing from
the 60 Raptor kit. Definitely do this.
Switch the pipe -- the MPII is designed for 90 size helis with
gear ratios in the 8.x range. Hatori makes a muffler that
works well for 70's.