I believe you can do what you want in Motion with no problems. It offers 5 retiming behaviors which can be applied to part or all of a clip. Read the manual. exert below.
When a clip is selected, a group of Timing parameters appears in the Properties Inspector. These controls adjust some of the same effects as the Retiming behaviors, such as slowing down or speeding up, looping, or reversing a clip. However, the Timing controls affect the entire clip. The Retiming behaviors, on the other hand, let you define which portion of the clip is modified by a timing change. Any timing changes made to a clip using the Inspector’s Timing controls are respected by the Retiming behaviors. For example, if you changed the speed of the clip to 50% in the Timing parameters, a Retiming behavior uses that half-speed clip as its source. For more information on Timing controls in the Inspector, see Retiming overview.