Mate, what orion you got? ill be able to tell you what fits, ive changed a few shocks on orions. Same as most pitbikes, 27mm from eyo to eye on most orions, and 30-35 on the AGB-29.
That would be just a standard cheap "pogo stick" shock in that bike, replaceable with a shock that is 27cm from eye to eye, Id recomend fastace, got one of these in my agb-29 as stock and its superb!
It may not fit. I've seen a batch of those with 200mm shocks. They have the top shock mount welded on way low on the frame.
Delza125 you'll have to give us a measurement from the middle of the top bolt to the middle of the bottom bolt on the shock, with the bike sitting up on a stand or bucket. As long as the rear wheel is off the ground.
Oh ok, well now it depends what you want to spend. There are ebay cheapies for $80-$100, or some slightly better for the high $100's or really good shocks for $200 +. I have used this shock before and found it very good, with rebound adjustment and for reasonable money.