I think the Thermostats that start to open at around 104°C are the highest rated ones available.
Mine's a Moshimoto item, starts to open at 103°.
All the 1200 8V Engines are very over-cooled, and any Air-Cooled Engine with an Oil Cooler should really have a Thermostat fitted in the Cooling Oil Circuit.
Can you imagine the outcry if a manufacturer released a car without a Thermostat in the Cooling System!!
In cooler Weather/Climates, the Oil probably won't reach a decent temp in normal use.
Synthetic Oil works best at around 110-120°C
In Ambient temps around 14°C (57°F) or lower, the Oil won't get much above 70-75°C (158-167°F), which isn't good for the Engine or the Oil, you'll get emulsification of the Oil, which can lead to internal corrosion.
It's worse on the later (post-2011) Stelvios, with the larger Cooler.
It's worth fitting an Oil Temp Gauge to keep an eye on temps, especially in cooler weather.
I made my own Thermostat Housing, just used the Moshimoto Waxstat, it's the same part used in off-the-shelf Thermostat Units.
Machining my own Housing meant I could reuse the original Cooler Hoses, and the Unions, they're not standard Hose Fitting items.
Using an off-the-shelf Unit, you'll need a few more Fittings, and possibly some Hoses.
Have a read through Mark (Beetle) Bayley's write up of fitting a Thermostat to his Griso 1400.https://www.grisoghetto.com/t4478-8v-overcooling-project