good question. i greatly appreciate that you want your bike to run at it's best. without jetting, you can cause damage to your engine if the bike is running too lean from a lack of fuel. with your new parts installed, your engine is breathing much more air. so this means you must change your fuel jets in the carb to bigger, to add more fuel.
the stock jetting in your rm250 k1(2001)depends on the california model or not. if it is a california bike, then the pilot jet is a #48, and your main jet is a #162. a non california bike has pilot jet #168.
either bike you have, the jetting would be adjusted the same method. for the next step, you need to buy a jet kit. there are different brands, like dynojet and moose racing, but to properly jet your engine you need a complete jet kit. the difference in the jet kit to replacing the jets are a few main reasons: the jet kit comes with many more parts then just different jet sizes. it will come with a new fuel needle, to richen the mid range throttle speed up, a new slide spring, many pilot and main jet sizes, and a few air jets and clips for the jet needle.
the choice of the jet kit is up to you, but i highly recommend dynojet kit. they have the most research and development compared to all other companies.
inside the jet kit will include instructions for the parts replacement, too. it will tell you how to replace the springs, needles and jets. there will also be a jetting guide for you. this will tell you how much to rejet the bike. because you have a high flow intake system, and a high flow exhaust you would want to rejet the highest setting the kit comes with, which would be the #55 pilot and 178 main jets.
so, your next step is to choose a jet kit, and replace the parts needed for your carb.
good luck, ride safe !!