make sure you wire up the fan relay somehow. Titus aren't showing that. the fan will run 24/7 on this if it's just line voltage. typically you only want to run the fan when the AHU is running or during a night heat load. without the control the fan run hours will be 3 times what they would be without. across an entire facility that's huge.
I'm of course saying that because the drawing is titled "Series Fan Powered Cooling w/ Electric Heat". I'm assuming that's actually what you have...
Nice. Didn't notice the date on the drawing. The interface between the VAV controller and the wall sensor looks non standard. If you can't find the stat you could just replace the VAV controller with something new. Shouldn't your Titus rep help you out here? I'd like to know if they're not standing behind their old stuff.
I think a lot of older VAV box controllers have proprietary interfaces with the stuff that goes on the wall. Basically any analog electric VAV controller will be non-standard in some way. That 16 VDC power supply out to something on the wall is definitely non-standard. A new DDC VAV controller would probably have 24 VAC and communication (whatever language it's speaking) to the stuff on the wall or 24 VAC and AIs for set point adjust and space temp and maybe a BI from a configurable momentary switch on the wall sensor. The controller at the box then decides what to do with all that information. That would have been the only "off the shelf" way of dealing with this since you'd be able to get a new VAV controller and wall control "off the shelf".
Hope it works out ok and doesn't cost you too much money.