Assuming that you already have the spawn part (let's name it SpawnPart) and the robot (let's name it Bot) are all in the Workspace and already set up.

Now, there are two concepts you will need to understand:

• Regeneration

• Finding the distance between two objects dynamically

REGENERATION

--Server Script
Bot = workspace.Bot
SpawnPart = workspace.Spawn

BotCLONE = Bot:Clone() -- Cloned Bot to reference the most unaffected model; I'll show you why in a bit.

function RegenBot()
    if Bot then
        Bot:Destroy()
    end
    Bot = BotCLONE:Clone()  -- I cloned the clone. The reason why I is because BotCLONE will basically be the epitome of all Bots, because the original Bot will be deleted when this function is executed.
    local Torso = Bot:FindFirstChild("Torso")
    if Torso then
        Torso.CFrame = SpawnPart.CFrame     -- This will teleport the Bot to the SpawnPart, just in case if the spawn moved to a different position.
    end
    Bot.Parent = workspace
    -- The default Parent of a cloned object is always "nil".
end

DYNAMIC DISTANCE

-- Same Script
Bot = workspace.Bot
SpawnPart = workspace.Spawn

Distance = 20

function CheckBot()
    local Torso = Bot:FindFirstChild("Torso") -- This is needed if you do not like to see errors in your output. Sometimes the function runs, even when Torso is not defined yet due to the clone process.
    if Torso then
        if (SpawnPart.CFrame.p - Torso.CFrame.p).magnitude > Distance then
        -- If the distance between the SpawnPart and the Torso is greater than 20, then the Bot will respawn.
        -- General formula: Final Position - Initial Position (it doesn't really matter since we're dealing with magnitude)
            return true
        else
            return false
        end
    end
end

while wait(2) do    -- I made this check between a long period of time (usually, you'll see the wait time to be 1 second or less), because having the script check the distance between each small amount of time (e.g. 1/30 of a second) is just too much game activity for something so trivial in my opinion.
    if CheckBot() then
        RegenBot()
    end
end

Any questions/comments/skepticism? Ask away.