You can probably do everything you need to do with a dozer and an excavator, even if they are small.
How big around are the trees at 4-foot above the ground level? If they are less than 4-5 inches at that height, I'd pull them out, roots and all, with the excavator. Then just smooth it and rework the surface with the dozer.
If you have to re-do the dam, I'd still just take out the trees, but leave most of the dam. You can re-core it by starting a trench on the water side of the dam with the excavator. Then push clean clay into the trench, while compacting compacting the clay into the trench (core) with the dozer.
Unless you've got some really big equipment, you shouldn't have any problem getting somebody to move them for you for a reasonable price. I'd check with the neighbors. We regularly move a big skid steer, an excavator, and a 68 hp tractor (not all at the same time) with a 1-ton pickup and 5th wheel trailer.
Ken
