Depending on how accurate of a number you're looking for, as it's always more complicated than it seems it should be with Nortons...
There were varying fin and exhaust angle configurations on 88s, always trying to keep the head cooler. Also, there's the variability of when they changed from a 3 start to 6 start oil pump, always in search of keeping the oil/head cool.
Which head are you talking about? Is it the SS head you're working on? If so, it would be pretty comparable to any of the later bikes, 650ss, Atlas, and even the early Commandos, depending, of course, on how much stick you intend to give the bike.
Long story short, there's a temperature range given to heat the head to pull the valve guides out (150-200C?), so using the low end of that spectrum as a top end of the head's operating temp might be a good place to start if you're looking for a usable range for an industrial adhesive or retainer.
