Move/copy the ppcross8086.exe to the same directory as the ppcx64.exe and move/copy all the units from your i8086 installation's units directory to your x86_64 installation, then you can select the i8086-msdos target from within Lazarus. (It doesn't matter that the ppcross8086.exe is 32-bit, cause it's just the Lazarus IDE calling the executable and MS-DOS programs won't grow that big that the 32-bit boundary is a problem anyway...)