Did you look into a benjamin XL1100 gas piston they're 18mm and body length is 6 5/8 and total length is 11 3/4 long ,might be longer just replaced mine and the new one was tad longer. BT1500-00-2 from crosman called gas spring.
This isn't strictly related to the topic, just an observation.When I received the basket case Octane, the trigger assy. was completely disassembled.I had thought it just to be a cruel joke, but now understand the situation.The Octane trigger assy has 8 pins IIRC that all the parts pivot on.Other than the latch sear, there is no means of keeping any of those pins in their proper locations, except the cavity in the stock.Remove the mechanism from the stock and lay it on its side, and those buggers will fall out!! (Good ol' gravity doin' its thing...)5 of those pins are 3mm dia. I am considering making some pins slightly longer and turning E-ring grooves in them.For the time being, when I have the action out of the stock, I will place some tape over the pin ends in an effort to keep them from falling out.....
Nice job with the conversion!!!