I would replace altogether ... but how about carefully cutting the entire flat part of the bottom and replace with another insulated flat material ... certain glues might cause a problem but two side tape or the right caulking might work instead of glues or epoxy??
Check what brand and model it is.
Some manufacturers sell replacement foam bottoms and tops.
If all the inside stuff is in good shape (cords, heating element, thermostat, and so forth), then it is cheaper to just buy new foam to put them in, instead of buying a whole new incubator.
Find a portable flat insulated material roughly the same size as the bottom of your unit and caulk the unit down to it that surface and fill the hole in the side as well