The bolt pattern is different because the mounts are different.
Welcome to the other side of the hobby. While learning to fly is important, also important is learning to repair. One of the facets of repair is sometimes modification. You can mount the 'LA on the plastic mount by drilling the holes for the engne you wish to use in the right pattern on the mount rails. With the AWARF, you also must fit the throttle rod. These items are not prefitted or drilled so that you can fit the engine of your choice to the model. Fit the engine to the rails with 6-32 screws and nuts. Use a bit of loctite. You can also use the old mount if you wish. You will just have to fit it to the new firewall and drill holes as needed. Install new T-nuts and maybe plug the unused holes with a bit of scrap wood glued into the holes. Seal these wood plugs with a bit of epoxy for fuelproofing. Talk to the guys at the field. They'll teach you.