It would be a fair bit of work, but not on the level of near-total rewrite. The main question is how to handle levels that are too small to make use of additional area. Lix handles this by putting a border (in a color very distinct from the background black) in the unused area, while the skill panel still takes up the entire width. This is somewhat ugly. It is also less practical for NeoLemmix, where level backgrounds are not always the same color. Not expanding the skill panel when this happens would then mean the user interface varies based on the level size, so this is not ideal either.
Stretching the skill panel by a non-integer multiple is also going to produce some somewhat ugly results. The alternative to ugly results is to use a TLinearResampler on them, which looks much nicer but requires much more processing power. An option for this exists in current code (newer than the V10.12.14 release) with existing zoom mechanics, with seperate options for menu screens and in-game (there is no seperate option for the skill panel vs the gameplay area); this defaults to on for menus, and off for in-game. On my PC, in-game performance is the same at normal speed with or without it, but fast forward is significantly slower than normal. I suspect that applying this only to the skill panel will reduce, but not entirely eliminate, this issue. More complex solutions involving pre-rendering the resampled graphics may exist, at a cost of much higher memory usage. This is probably the optimal solution, but also the most work.