Not really necessary in this environment. Designed for cold weather (like snow and sleet and sub zero). If its full open to the snorkel blocking the heat riser won't hurt anything. The air motor only kicks in in very cold conditions, under hard acceleration the air motor will override the thermal bulb, if freezing air is sucked in...
There are firms that repair them, I tried one for my 68 S code, average work and expen$ive. NOS are available from time to time, but again expensive.
As yours is full open always, it may just be a misadjustment on the thermal rod. It screws in and should open with about 100F temps. I've tested quite a few for my 390's and they seem incredibly reliable. I'd test that setting first. If that works, as noted the air motor is secondary and having it full open to the snorkel is better than full closed...