Those 6V 5Ah sealed lead-acid cells look like the correct size.  Two in the head on either side of the sonar system, two between the wheels?  And what kind of operation time have you been able to get from your Hero 1 as a result?  I think we might have a winner.

Actually 3 down low between the wheels, for center of gravity, and one up higher to counterbalance the weight of the arm. The original batteries were about 6v lantern size, 2.5"x2.5"x4", 4Ah. The batteries I have are that size but 5Ah. The ones I referenced earlier on Amazon are rectangular, very slightly wider at 2.8"x1.9"x4". I'm not in front of the Hero now, but if recollection serves that should be well within the available space in all locations. As far as performance, we haven't gotten far enough with our restorations yet to push the battery capacity. Coming soon... ;-) -Corey L. Curator, cpmuseum