Anywhere Fleet using SDK 3.5 (old game that can't be upgraded to GameLift v5.0 SDK)

I am operating an old game that has entered the "long tail" of its life cycle. At some point the GameLift EC2-managed server costs will cost more than the game makes in revenues, which means the game will get shut down. GameLift Anywhere fleets would solve that.

I have not been able to successfully upgrade the GameLift SDK from 3.5 to v5.0. I've spent way too many hours on this, so I wanted to shift gears to see if something like the following might work:

GameLift SDK versions older than 5.0 only had InitSDK(), which expects there to be a GameLift agent running on the host. This agent is provisioned by GameLift services when the EC2 instance is created. Is there any way I could run this agent on my own hosts? Could I maybe proxy requests to a GameLift endpoint?