The pipe the Shark Shifter folks sell will work fine, we’ve used it on ours. The 2003 engine fits in the earlier mount but it makes the carb sit at a slightly steeper angle. The 2002 and 2003 engines have an electric power valve that you can disconnect and put stop screws in the cable holes to hold it open. You’ll never drive below the speed where it works anyway. You will need to keep all the electronic boxes on it so the ignition will work. Don’t worry about porting it. We have one that is and one that is stock. The modified one is faster but the stock one ends up nearly as fast and is way easier on parts and to drive. All the carb things that work for a 125 Honda will work on the 250 – except that you can go bigger. Ours runs fine on a 38mm but LIKES the 41mm. Fills the top part of the power band in nicely. Keep the rpm to 10,000 or below and it will have a long and happy life. Best part of the power curve is 8000 to 9500 anyway. Gear for about 10 mph higher than what the fast 125’s are doing for starters. The hardest thing will be is to keep your self from driving it like a 125. You’ll just burn the tires off of it and go barely faster than a 125. Baby it off the corners is the fast way. Another last thing to look out for is vibration. They shake differently than other kart engines (harder and at different speeds) so you’ll find that different stuff falls off. Keep close track on all the nuts and bolts until you figure out what’s what.
The last thing is I would reconsider not Road Racing it, they do great! Enough power to really go and enough room to “stretch its legs”. It’s a blast!