If your SQL installation is under .NET 4 and using the latest version of mojoPortal 126.96.36.199 your plan is sound assuming the SQL CE database is correctly produced by the downgrade tool.
If you are running an older version you should upgrade first to mojoportal-2-3-5-1-msql-net40-deploymentfiles.zip - hosting must be .NET 4
Then you really only need to replace the mojoPortal.Data.dll and mojoPortal.Features.Data.dll with the versions from mojoportal-2-3-5-1-sqlce-net40-deploymentfies.zip
Then you need to put the name of the .sdf file in this setting
<add key="SqlCeApp_Data_FileName" value="mojodb.sdf" />
However if you get errors in a downgraded database I can't really help you unless you can produce the same error in a SQL CE database created by mojoPortal. I cannot be sure that a downgrade tool will leave the SQL CE database in a correct condition. If it works great, but your mileage may vary.
Note also that if you need to use the WebStore feature, that is not supported in the SQL CE version of mojoPortal and there are no immediate plans to implement support. For ecommerce I recommend stay with SQL Server.