There is a search function for IRV2, look in the upper header. Do a search for the Howard Control System
Unfortunately there is not much information out there and appears that most of parts are obsolete, but you may get lucky.
Here is a link to a complete manual that might be of some help.
https://janeandjohn.org/docs/Howard%20PCS%20Manual.pdf
If your cylinder is leaking chances are it is just the gland rod seals that are bad. The gland seals usually consist of Oring and wiper type seals. Pretty simple design. A lot of times these are common sizes and a good repair shop could possibly rebuild, as long as there is no internal damage. The cylinder will have to be removed and disassembled to repair.