How to protect the early stages of a proof-of-work application?
... insert fraudulent transactions ...


No, fraudulent transactions and blocks containing them are rejected when nodes follow the consensus rules.

There is a greater risk of chain reorganisation, meaning miners who successfully mined a block may not eventually have the reward for that when their block is no longer part of the longest chain.

The great risk of a 51% attack is that the attacker could outpace the rest of the network to assert that the attacker's version of the blockchain is longer, re-writing history with valid transactions and claiming block rewards, or undoing their own transactions when re-writing history by spending the utxo's back to themselves first, making their previously valid transaction seem like a double spend, for example, and stealing from a merchant.

To protect a new proof-of-work application it is only necessary that the work is reasonably distributed. For example, in the case of a Bitcoin full clone the same ASIC miners would be valid, however, the cost-for-benefit would be very low initially and so an attacker must ask, what benefit do I derive?