Well, you do not have to use a Yamaha controller for throttle / transmission function. But you need something be it Sea Star solutions or a Chinese knockoff from Amazon. (BTW- I put a knockoff on a friends commercial boat and it works fine for much less money.)
If you have some proficiency with wiring you can add a stop button (or toggle switch) and an ON/OFF/ON single pole double throw (shorthand referenced SPDT) toggle for the tilt function. There are about 8 or 10(?) wires in the wiring harness and you can pretty readily find the wiring color code and schematic online for a T9.9.
The "T" prefix indicates it is a high trust model, the "L" for "long shaft"(20"), and the PB I believe is for a remote? But I cannot be sure as my memory fades. I have owned dozens and used them commercially for years in a past life. BUT- They absolutely HATE ethanol fuel.It will mess with the low speed jets in a matter of weeks and will not run much below 3k RPM. So if you must use ethanol fuel- run it often to keep it from varnishing the jets which is a pain in the ass to clean the carbs. Last tip, on the side of the carb by the throttle butterfly actuator is a little lug that has a (convex I think?) cap at the end of it. Take a very small, sharp flat blade screw driver and tap it through the cap and then pry it out. Under that cap lives the air fuel mixture screw the EPA does not want you to touch but will terrorize you with stalling and idle headaches for the rest of time if you do not adjust it on occasion.
They are great little motors otherwise, but what I have said has been true for over 20 years and every one of them I owned. (Oh- and lube the transmission and throttle cable a few times a year at the engine or they can get gummy and hard towork- mostly the transmission actuator cable.)