Make existing android app Cross platform and make it available on Playstore and Appstore

I have an Android app. It is well working for nearly 2 years.
Code will be given. Java code and mssql queries are ready.
Test database will be given. It has ready to use data.

Now I have following requirements:
1. Make it available on Playstore. Now its older version is available on Playstore and I cant change it, it's so confusing.
2. Make it available cross platform (Android and iOS) using "Xamarin".
3. Make it available on Appstore of iPhone. Appstore's year fee will be paid by me hhe.

App have to be able to run on Android 8.0 or higher and iOS 12.0 or higher.

In the end, I will get Xamarin code.