If we clone ethereum from Github can we change its consensus like other parameters Gas ,blocksize. like if we dnt want to build Blockchain from scratch is there any way to change the consensus mechanism. which platform is best for it
As long as you are talking about
go-ethereum you can indeed choose to run your node with PoW or PoA (Clique) consensus engine just by tweaking
You can learn more about running PoA nodes by following this giude.
You can also write your own consensus engine by implementing the Engine interface and passing your implementation in backend.go, worker.go, blockchain.go, etc. In that case I would advise you to thoroughly learn the inner workings of the existing consensus engines and their interactions with the rest of the system. You won't be "building blockchain from scratch" but it honestly takes a lot of time and effort.
Apart from choosing one of the two existing engines (PoW/Ethash, PoA/Clique) you can build your own algorithm or modify the existing ones, but that takes time.