I'm trying to start a new genesis with v12 block version.
v1 as is explained in depth here
works without problems.
What info / bytes are needed for v12 different to v1.
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"v12" refers to the hard fork version, not the tx version. Given that, and by the mere definition a genesis block is the first block mined, what you're asking makes little sense - compounded by the fact you talk of block versions but are actually referencing the miner tx of a genesis block.
If you are trying to fork Monero with a new chain that starts with v12 functionality, it's not the genesis tx that needs changing, rather a bunch of other code.