Can you use MouseEnter() on a physical part??

As an alternative to ClickDetectors, you can use a combination of an event, UserInputService.InputChanged, and the Mouse.Target property:

local uis = game:GetService("UserInputService")
local Mouse = game:GetService("Players"):GetMouse()

local lastObject
uis.InputChanged:connect(function()
	if lastObject ~= Mouse.Target then
		if lastObject then
			--MouseLeaveFunction(lastObject)
		end
		if Mouse.Target then
			--MouseEnterFunction(Mouse.Target)
		end
		lastObject = Mouse.Target
	end
end)

InputChanged fires when the Mouse moves or scrolls, or wherever a mouse&keyboard input doesn't have a definite 'on' or 'off' state (such as a Key being pressed or not).