Porting to any platform is trivial after understanding the basics as described in these articles

The author explicitly describes why they chose riscv32

Why would you want the author to use riscv64 instead?

At least on Intel, it was quite a significant difference in some ways, in kernel terms.

No it's "left as an exercise to the reader" ;)

But seriously, it's not that hard. Change build options to generate 64-bit ELF, replace all 32-bit-wide parts (e.g. uint32_t, lw/sw instructions), and implement a slightly more complicated page table (e.g. Sv48).