Modern Vespa

I have a super with a GL/Sprint euro-style headset.

I'll be swapping out the 8-in front fork with a Stella fork for the 10-in rims and fully hydraulic disk brake.

I need to mount the master cylinder on a GL headset.

Is there an adapter online that I can buy that would mount under the headset to hold a Grimeca/LML/Nissin/Heng Tong style Master Cylinder.

have a look at these, I think there are a number of us on here that use them

https://www.sip-scootershop.com/en/search/brake%20master%20crimaz

Though this wasn't your question, but you're going to have a real challenge fitting a Stella fork under any scooter that doesn't have a PX/T5 mudguard. A PK fork is challenging enough to do. But with a fair amount of sectioning out the mudguard, anything is possible. I just wanted to throw this out there.

Think this is what you are asking for -

https://www.sip-scootershop.com/en/product/brake-pump-holder-mmw-brake-master-cylinder-grimeca-98-handle-bar_56377500

If you don't want to cut the brake lever perch (think that's the right word) off the headset then go for one of the Crimaz master cylinder setups.

If you're going to cut a hole in the mudguard for the top of the shock/mount/fork, it's pretty clear how to go about that. Have a look at every Scooter and Service conversion.

If you don't want to cut the mudguard, consider first using a Rally guard (bigger than a Super). Even that may need bulging to some extent. When I did one for a mate there was the choice of 2 guards, one had internal brackets that I had to partially cut off.
Lots of info from my conversion for fork and master cylinder from a few years back here - Fork Swap for 150 Super