In regards to MWO, The game's set for mid 3049, so right at the start of the Clan incursion into the Inner Sphere. It's safe to assume that 'Level 2' Technology will be the Norm, so Gauss Rifles, XL Engines, Pulse and ER lasers, Streak SRM systems, and, of course, Ferro-Fibrous armour and Endo-Steel internal structure are all available to be played with.
So if you really want to, you can swap out the Jenner's engine for an XL version and have some free space to play with. (Personal Suggestion? 2.5 more tons of armour) The mechlab system lets you modify and upgrade your machine of choice in a variety of ways, which I'm glad for.