So I recently decided to get into modding, and as of today, I'm still very uneducated. I deleted the original mario spawner and I was wondering if I could access it again.

You have to place an entrance and set the view of it correctly.

Also be sure that it's id matches the start id in the general level settings tab.

If you got problem with it have a look at existing levels and how it is done there.
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GitHub - Kuribo64 - YouTube