The Prison is a Chop Shop
Chop shops are a prefect job for pirate slave labor. It is labor intensive, a bit hazardous, and is necessary for pirates to be able to turn the scrap from uncompliant victims into profits. And most importantly, it gives you a good reason to give your prisoners direct access to vehicle equipment without it being an obvious security oversight because all the stuff the prisoners are handling would be seen as non-functional in the eyes of the pirates.
It’s a tool or equipment that pirates logically would have stolen/acquired for their own use (rather than for hawk - items with trade value trade are more protected and harder to explain how he accessed it)
This part you have backwards. It is security risks, not value, that the pirates need to be mindful of. In a prison system, all vehicles are considered high security risks; so, the guards would take every precaution to keep prisoners away from any fully functioning vehicle. However, pirates will often have access to parts that need serial numbers scrapped off or some minor bullet holes patched up before they can hawk them. So instead of having access to a vehicle, he instead has access to valuable parts that are individually seen as low security risks.
The the air tight cabin could be the cockpit of vehicle the pirates shot down that the prisoners have not fully cut down into spare parts yet. The balloon section could be from an airship they shot down that the prisoners have been tasked with sewing back together. The Helium could be from the welding equipment (because you need a noble gas to prevent welds from oxidizing). And the propulsion could be anything from that stack of spare motors in the corner that still need thier serial numbers removed before they are ready for sale.
This idea of vehicle made from spare parts would fit especially well with:
He will almost certainly make the escape by improvised flight, which means he will be hanging from some spare ballonnets and using something for directional motion. Probably won’t have time to rig control surfaces and none need to be in the donor vehicle. Just brute force propulsion.