Try to get them to tell you exactly what they don't like about A7. Have them write it down onto a list, if you have to. Then come up with arguments that will defeat their assumptions, and hopefully they should listen to you. Once you've got them listening, explain to them how you can get results right away with A7 since you already understand it, whereas using another engine would slow the project down significantly. Convince them to try A7, if only for a little bit, and if they like it, then you can stick with it for the rest of the development cycle.

If they won't agree with you, then all you can do is either switch to another engine or leave the team.