I have several mechanisms based on the old Front Range frames. Note that none of them use the original trucks, as they were extremely poor in quality. You may be in for a lot of frustration if you try to make them function reliably.
The good news is that Athearn parts drop right in. You can use Athearn GP trucks with no modifications. I know you said you have a motor, but for anyone else reading this (or if you change your mind), Athearn's RTR or later blue box motors will work, I think right down to the stock motor mounts. Repower kits from A-Line/Proto Power West can be installed easily too, and can readily be made to work with Athearn universals.
You can also substitute a Kato motor, again using the stock mounting bracket and screws. If you can get them, Kato trucks from the old Kato manufactured Stewart F units are a drop fit just like the Athearn trucks. On all of my Front Range Geeps, I installed the entire mechanism from a Kato F unit, which fit like it was made for the Front Range frame.
Bottom line, decide on the universals after figuring which replacement trucks you want to use.