How would I regen this model in the same position after making it fall and dissapear?

Well, once they fall into the void, they're gone.

Your script is a solid base, though.

local Model = game.Workspace.Stairs

for _,v in pairs(Model:GetChildren()) do
	local debounce = false
    if v:IsA("BasePart") then
        v.Touched:connect(function(Hit)
            if Hit.Parent and Hit.Parent:FindFirstChild("Humanoid") and debounce == false then
				local stairclone = v:Clone()
				stairclone.Parent = Model
				v.CanCollide = false
				v.Transparency = 1
				debounce = true
               stairclone.Color = Color3.new(0,0,0) --Color turns black
               wait(0.3)
               stairclone.CanCollide = false
               stairclone.Anchored = false
				wait(5)
				v.CanCollide = true
				v.Transparency = 0
				debounce = false
            end
        end)
    end
end

It clones the stair prior to it falling and regens 5 seconds after falling. There is also a 'debounce' put in, for each part. If you have stood on the part. It will activate the debounce so it won't keep creating clones. As the for loop will not update to see a clone, the clone is merely the falling stair, whilst the actual stair is turned transparent and non-collidable.

I hope this helped.

Thanks,

Explosion