They have every right to voice their opposition, if he's being too moderate, then they should be preparing to find someone to represent their values better. The Republican Party is the conservative party, and they should act as such.
We get you want the GOP to lose, but obviously Baker is the most conservative possible governor you could elect in MA.
We have seen a different model. Mitt Romney ran as a moderate technocrat, but then governed as a conservative and a partisan Republican. Charlie Baker ran as a moderate technocrat, and by all accounts that's exactly how he plans to govern.
If the "real Republican party" wants to coalesce around Curt Schilling or Brian Camenker to run, they're welcome to try.