As regards sealing your pond, the liner option from a reliable company should serve your purpose.
A pond that is 100'x125' is about 0.29 surface acres. Your evaporation losses in Michigan would not be that great over the course of a year. If you already have a water well on the property, you could grade the land around the pond to exclude all surface ground water from entering the pond. That would keep the pond cleaner, regardless of whether you installed the pond liner or tried to compact clay blankets around your pond leak levels. Your pond would then have to operate on well water, plus the very small amount of direct rain or snow that fell into the pond.
If that is going to be too expensive from either electrical costs or water usage restrictions, then you could do some re-grading (while you have equipment on site for the other work) to direct water into the pond through a wide, evenly-graded area that has good ground cover vegetation in place to "filter" the incoming rain/snow water.
Finally, if you (or one of your neighbor's) knows a good pond builder in the area, you might also get a bid to disc or rip your existing leaky banks and then build a compacted clay blanket over those pond banks in several 6" lifts. You might be able to do that for less than the liner bid, but the liner would probably be closer to a "sure thing". (There are no 100% sure things when building a pond.)
