My non permanent light up Thomas attempt worked exactly how I thought it would, with little to no issues. Even both wires are thin enough to run under the splasher and allow the shell to fit normally. In a very convenient manner, the switch contact on the drive unit is big and exposed and I didn't have to dismantle the unit, making it almost laughably easy. All I had to do was make sure the wires were touching the negative terminal and switch contact (no need to wrap them either), and it turned on. I did have some trouble getting the shell back on without one/both wires moving away, but they stayed put after a few tries. Pardon the blue wire sticking out.
Under the headlamp happens to be a small casting hole, which I don't recall seeing on the TOMY/HiT chassis. If I had the guts to make this permanent (and had multiples of certain engines for modding), I could've made that hole big and deep enough so the bulb could fit under the lamp. Unlike most owners, I'd prefer to use my electronics in scratchbuilds rather than mods for commercial items. Either way, the mere fact that it worked with "zero" effort was enough to get me excited.