JH motor club on YouTube has a 2018 EFI KLX 250, and has done a big bore + lidless airbox. There are a number of videos with useful information.
For anything 330cc and up, an upgraded/larger fuel injector may be necessary. Otherwise, the injector duty cycle may end up very close to its limits.
Both BEET and Keng Engine offer upgraded injectors.
Bill Blue offers a customized fuel controller for the 351 kit, which is likely to be used w/ a stock or lightly modified intake and an aftermarket exhaust.
Dynojet PCFC made for the KLX 250 aren’t sold in North America, but Dynojet does sell them in Europe and eBay sellers will ship them worldwide.
Keng Engine appears to offer pre programmed API tech aftermarket ECUs. ECU Shop also manufactures standalone ECUs, and they list the DTX(D-tracker X/KLX 250) as “coming soon”