Create a blank new MPLAB X project as normal, selecting mpasm as the toolset.
Add the project .asm files to the new projects “ProjectName.X” directory.
Right click the project > Add Existing Item > Select just the main asm file and add it to Source Files.
Check the assembler command line switches you used for the project in MPLAB 8, right click the project in solution explorer and select Properties > mpasm > mpasm and make sure the settings are set the same here.
You may need to disable case sensitivity if you used to have it on and now capitalization is different with MPLABX system files – can just turn it off if so.
Build the project