If you can get a 10oz tank, that would probably be perfect. My ASP .40 gives me 20 minute flight times (65%-85% throttle) with an 8oz tank. Your Super Tiger .61 will consume fuel a bit faster.

Also, is the current tank round or square? Different shape and different manufacturer will change the dimensions. You may be able to use a 12oz tank, but it should have sufficiant clearance to pad it properly with foam. I recommend having at least 1/8 inch (preferably more) of padding on all sides.