You can still create a login with 'sysadmin' access. Thanks to Principal Data Engineer at Microsoft (Saleem Hakani) had authored the below on *SQL Server Tips & Tricks*:

I found this while I was stuck in similar state when I forgot the sa password for my test machine.

https://docs.microsoft.com/en-nz/archive/blogs/sqlman/tips-tricks-you-have-lost-access-to-sql-server-now-what

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> Here are the steps you will need to perform:
> 
> 1. Start the SQL Server instance using single user mode (or minimal configuration which will also put SQL Server in single user mode)
> 
> From the command prompt type: `SQLServr.Exe –m` (or `SQLServr.exe –f`)
> 
>  
> 
> Note: If the Binn folder is not in your environmental path, you’ll
> need to navigate to the Binn folder. 
> 
> (Usually the Binn folder is located at: `C:\Program Files\Microsoft SQL
> Server\MSSQL10.MSSQLSERVER\MSSQL\Binn`)
> 
> 2. Once SQL Server service has been started in single user mode or with minimal configuration, you can now use the SQLCMD command from
> command prompt to connect to SQL Server and perform the following
> operations to add yourself back as an Admin on SQL Server instance.
> 
>        SQLCMD –S <Server_Name\Instance_Name>
> 
> You will now be logged in to SQL Server as an Admin.
> 
> 3. Once you are logged into the SQL Server using SQLCMD, issue the following commands to create a new account or add an existing login to
> SYSADMIN server role. 
> 
> To create a new login and add that login to SYSADMIN server role:
> 
>       1> CREATE LOGIN ‘<Login_Name>’ with PASSWORD=’<Password>’
> 
>       2> go
> 
>       1> SP_ADDSRVROLEMEMBER ‘<Login_Name>’,’SYSADMIN’
> 
>       2>go
> 
> To add an existing login to SYSADMIN server role, execute the
> following:
> 
>       1> SP_ADDSRVROLEMEMBER ‘<LOGIN_NAME>’,’SYSADMIN’
> 
> The above operation will take care of granting SYSADMIN privileges to
> an existing login or to a new login. 
> 
> 4. Once the above steps are successfully performed, the next step is to stop and start SQL Server services using regular startup options.
> (This time you will not need `–f` or `–m`)