Roller bearings and extremely high cylinder pressures don't mix well. Unlike a shell bearing that spreads the load across the entire surface of the bearing and cushioned by a high pressure film of oil, the load on roller bearings gets focused to a few small points where the bearings touch the races. Needle bearings do a little better since the rollers are cylindrical, but it's still nowhere near ideal.
Ignoring that issue, the rods these cheap engines come with would fold in on themselves like they were made of cheese, or snap in half like a toothpick.
Diesel piston tops are reinforced more than standard gas pistons, and often shaped to focus the air charge to the center of the injector. They are also often forged units to better withstand the pressures and heat - why cold diesels are extra clattery.
Speaking of the injector. carburetors don't really work with diesels. Diesel engines have no throttle plate (generally speaking). They control RPM by altering the air fuel ratio, from say 60-80:1 at idle to 2:1 if the engine can supply the air to burn it. Since the compression is very high in the cylinder and the injection timing has to fire at right before peak compression, the fuel needs to be forced into the chamber at a very high pressure, and then that pressure needs to also be high enough to finely atomize the fuel for a rapid and efficient ignition even. This is why modern common rail injection systems are operating at tens of thousands of psi. You also need a pump system capable of generating that pressure.
