256, surely, if he wants to retain the advantages of 64-byte lines?
Anyway, 128×anything in mode 1 would make no sense.
Easier said than done with a two-fold difference in resolution; I presume you are prioritising number of colours rather than horizontal resolution, but that’s not “as straightforward as possible”.
don't forget Jonathan is much more familiar with the Spectrum than the CPC.
When he means "as straighforward as possible", it means in terms of core z80 code, and that the number of differences are limited to work out screen coordinates and finding the next line down, reading the keyboard and reading/writing files.
A lot is re-usable, and drawing sprites is about 90% the same.
So mode 0 version first then I am anticipating a mode 1 version to follow.
Behind AGD is already a thriving forum, instructions and existing games on the Spectrum.
This can all be used to help others to make games for CPC, or for them to remake the Spectrum games on the CPC.