I've had a couple great successes with a tender-mounted motor and driveshaft to the locomotive with a planetary gearhead on the Kato 12V 'critter' motor. But those little four-wheel guys have been a tempting and frustrating target as well. I've still got like three of them, and what's missing is Kato-style equalization and pickup. And most of them run way too noisy even if they are rather slow and powerful for their size. They do work, but you'd probably need a paired set in a tender, it's a lot to ask of one micro motor and one truck. I've done very well with the Solarbotics 25:1 planetary gearhead, but that's only ideal for my bitty geared steam with 10mph max speed anyway.
If you're looking at a kit, your dependence on a particular chassis part or motor is the weak link. I've done two different kits based on Tomytec parts, then they redesigned the truck gearbox and now it's really difficult to find them. I was looking at the Kato NW2 drive system for years and begged Kato to make them available to me, but they decided not to ever offer that motor and gearbox as a repair or supply part. That had potential, I agree, for short-wheelbase concepts. So did the Tomytec short TR01 and kin, but they are fast and relatively low-torque but at least 8x8 with flex pickups. I got an 8x8 Whitcomb drive concept to work with one of those. Works, well, sorta. Motors on those are B-grade.
You've got Kato, Atlas, Bachmann, and Tomytec that 'may' have parts and 'may' be a good source, if you pick something, don't be afraid to identify your idea to them and let them see you as a partner rather than a customer. My biggest disappointment has been the disappearance of Marsten (Gizmoszone) motors and planetary gearheads, they literally evaporated out business with no trace. And Eldon Shirley, 'Motorman' that had an absurd collection of lower-price Faulhabers. in 8x16 sizes.
One odd discovery was that the Bachmann 44-tonner chassis WOULD FIT in a USRA tender body. Now, try to find that mechanism on the open market, let alone the locomotive for an acceptable price. That was one of the best 8x8 short drives I've ever seen, but you couldn't mess with it much due to the frame design. The motor in those was the only double-shafted Kato-critter sized 12V I've seen, I got one, and dreamed of double gearheads on both ends for the ultimate 8x8, and then the gearheads poofed.