I am only a experienced user, not a developer.
So I contacted the guy who has compiled the TWRP for the Cubot X18 plus.
He has no time to compile TWRP for other devices, but he gave me the advice to try simply to change the kernel and that this is a zimage-file.
I downloaded the Android Image Kitchen
https://xdaforums.com/showthread.php?t=2073775, unpacked first the boot.img, than the X18-plus-TWRP.image, replaced and renamed there the kernel, repacked and flashed it.
It booted with the TWRP-logo - and that was all. The device was soft-bricked.
Or I made somethimg wrong, or the differences between Cubot X18 plus and Cubot X19 are bigger than expected, this way of "quick and dirty" modification didn't work.
So I stopped this activity and tried to flash the phh-treble-ROM.
But this is quite complicated without TWRP, beceause the Cubot X19 ROMS are buggy.
With the stock-pie-ROM the fastboot and the oem bootloader unlock doesn't work.
And the oreo-stock-ROM-recovery refuses to update zip-files (signature-failure) and adb sideload.
After a long weekend with a lot of trial and error I could flash the phh-treble-ROM (with the oreo-stock-recovery), but I couldn't flash zips and install Magisk or NanoDroid correcly.
So I have to go back to adapt the TWRP.
The kernels in the boot.img fo the stock-ROMs from oreo and pie have different sizes.
It didn't work with the pie-kernel, but I think that the kernel for the X18-plus-TWRP was oreo.
My last try to adapt the TWRP will be when I replace the oreo-kernel.
Or has somebody a better suggestion?
Best regards
Christopher