[Littlemaster]

When you lost your developer key, you must contact Google to sort everything out, they can't recover your devkey because it is encrypted so maybe they will offer you another solution.
Right now the only thing you can do without Google support is to notify your customers and let them migrate to the new application and meanwhile request a takedown for your previous application.

Good luck

Thanks for the response. Since it is an emergency time all over the world it seems google employees are also on leave. Support seems to be slow. Meantime I have just changed the package name and created a new project in google console. I have enrolled to google app sign and used my own key and received some certificate.
Currently the project is on pending publication while I unpublished the old app.