A servo reversing Y will not work.

Your options are:

1. Purchase an aftermarket electronic mixer.

2. Build in a mechanical servo sliding tray, I think Dubro even still sells hardware kits for doing this.

3. Split the surfaces up into three segments ailerons and an elevator in the middle with their own servos on each. This was Sop for many flying wing or delta wing kits in the past due to the lack of electronic mixers or the complexities of mechanical mixers.

4. Purchase an inexpensive radio system and I agree with carrellh the Hitc Laser 4 or 6 is a good radio with elevon and can be had used for almost nothing.

John