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You don't have to change Genesis block or change the code even. All you have to do is to change network by running bitcoin core in RegTest or the new SigNet network and mine there. In RegTest the difficulty will not change so it won't take hours to mine, you can mine hundreds of blocks in seconds.
I learn something new every day... This was the first time i heared about the SigNet... It seems like a "better" testnet. I have no idear how i missed this info
https://en.bitcoin.it/wiki/SignetRegtest is great for small scale tests, but the lack of a real "network" has stopped me from using it for large scale tests... It is, after all, a network in a single box...