Ducar/Predator 224 block comes with a 58mm stroke crank, they are also reinforced, the Tillotson block has even more reinforcing to prevent the neck and crank case separating. You'll need a billet rod, these are good for any abuse that's thrown at them. Any good Aluminium flywheel. Billet side cover for strength and double bearing support to the crank. Now you can add performance, the engine is basically an air pump, the quicker you can get air in and the quicker you can get it out the better it will perform, in other words gas flow the head, buy a gx160 head, they range from 16-18cc, this will up the compression without skimming. Install stainless valves something like 32/28. Warning you're now into dangerous grounds, 56lb springs will require molly rods, seats machined and you'll obviously install stainless followers with gage 1:1.3 rockers. The engine will breathe heavily so you'll need to install breathers from the block to rocker cover, then a vent to atmosphere, you can use a catch can or a simple filter, a one way valve is your choice. KEEP ANY PRESSURE OUT OF THE CRANK CASE Look at Pauls Karts or old MINIDRAGBIKE videos
Now you can start to get the engine to work, aim for 32-34BTDC ignition, PVL if possible for a stronger spark, clock the cool running spark plug if you want. Carburettors are a minefield, you need a good idle and main jet circuit with full adjustability, I don't run a carb, my engine uses fuel injection, some of the other guys will give you some good advice on carbs, although I'm more of a flat slide fan, Black66 has some videos on youtube for good carb setups. Next move onto exhaust, seems most use some sort of torque tube, Robertsons have good results, again I like a nice exhaust note, so I'm willing to sacrifice a little power for a sweet sound.
Enjoy the build, it's a great learning experience.