Your intentions of mating it to a T40 gear will limit your options quiet considerably. If you look beyond the transmission mating issue, you can go for a 7K (1.8L, available in both carb and efi), 5K, 3S-FE, 4E-FE, 4A-GE, 4E-FTE. The best option is the 3S-FE, it's fuel injected, can run successfully on CNG, easily available spare parts, good horsepower and torque.
All these would need custom mounts, a new gearbox (T50, W50, W58, etc) and matching diff.