Looks like you got a job on your hands there. But here is how I would start, get a 12 in peice of 3/4 PVC, cut you a slot in one end that would fit over that skegg. Then put a good wide as you can find hose clamp around it and motor housing about where that wire tie is. Then mount the trans ducer and bracket to that kind of out front of motor housing. Then cut any excess PVC off the end. Install a 90 degree elbow and a short piece of PVC into that pointing straight down long enough to protect transducer from under water objects. Then if it works and you work out any bugs and make adjustments I would then fab it up or have fab bed up out of aluminum bar stock. Then once you get that done I'd drill a hole through the slot and skag and install a smal bolt for support.