Hey guys i found a fix for you who doesn't have Steam version.
You have to open for example the file BIKE315.dds (rossi's bike, the original one) with a program , i personally use Paint.net is good for doing these things.
Then you drag the "mod" file BIKE315.dds on the paint.net screen where there's the old one and click on "Add level" , you save all and then do all the work with mixfile remixer.
hope this help someone of you.