Before you drill anything out, please show some close up clear pictures of the carb throttle assy.
I have a similar Briggs 8000 and 9000 series engines and have changed the carb that came with them as they were not used from gravity feed gas tank. The bowl with float valves in the carb kind of carb I am talking about.
I had modified the carb but only slightly to get it to have a twist grip throttle. I also am off roading so I use gloves and the additional grip that is on the handle I know is tough on bare hands, but splash with mud and you have a better chance with this type.
Back to the point, I have many documentations on my motorbike carb throttle setup in the threads I have started.
I did not like the plastic part of my carb that the connection would be made to, in fear that it would soon break. I also did not have any easy way to continue to have a governor on my engine. The reason is that I had to use and elbow manifold to clear the frame. I added a spring, separate from the throttle cable return spring. The spring is just connected to the butterfly valve plastic part outside of the carb. It is connected to the twist grip throttle assy by means of a pin on the assembly that connects to the spring.
With this modification when I am at idle or at full throttle, the spring bends. This way there is only very small force that can be applied to that crummy plastic part on the carb.
When I had another carb temporarily being used it had it 180 degrees the opposite side I needed to have the cable going to the carb. I reused without changing any existing parts, just added some new parts and removed others. The end result was the carb had other issues I did not like and returned it for warranty refund. I still had the parts saved and reverted back to what I originally had.
All in all you I know are creative and have something going that I like in that bike build.
A twist grip throttle not using any lock is a good way to go. There are those that use other means by small levers and the like to have a throttle, but they I have most notice have a return spring and do not lock.
https://motorbicycling.com/attachments/img07605-20120329-1804-jpg.46483/
Above link to picture shows where the pin connects to inside the vertical spring. That vertical spring is connected under a block of aluminum on a pivot point. The pivot point is actually from a door hinge. I added a place to connect a twist grip throttle cable and return spring. Also adjustment stops limiting both idle and full throttle.
Just figured I'd also mention that there is a block of aluminum I drilled and made to put between the manifold elbow and the carb since the engine is on a tilt and this levels out the carb bowl and float to be at its best.