i did some trading for an electric heli, now i am looking for parts but i dont know what are compatible. the heli is an mx 400 pro. ark or helimax??? im confused

They are all the same helicopter. Helimax was a distributor that rebadged the ARK X-400 as an "MX-400". Tower Hobbies discontinued carrying the MX 400 series a couple years ago, but still stocks replacement parts for it. ARK of Taiwan still sells X-400 kits online, a bit pricey in my opinion, though. Align Trex parts are for the most part compatible with the MX-400, with exception to the main frame and the main drive gear/ tail drive gear and pulley assembly. The stock main gear hub is crap, since the freewheeling unit is just pressed into the plastic gear and will slip on you. Helidirect sells a Microheli upgrade main gear with a hub assembly that is bolted into the gear. I still fly an old MX-400 with mechanical CCPM that has been upgraded with Trex parts, including a longer tail boom to accommodate standard 325 mm blades.

Hope this helps
Don