An easy way to cut down on unnecessary characters is to optimize your tracks. Optimizing your MML basically lets the editor reduce the amount of letters, numbers, and other things that your MML is composed of while keeping the song intact. In this case, optimizing your rank C MML would turn it into a rank D MML, because a lot of unnecessary writing was removed. You can do so by checking the Optimize MML box when you copy MMLs to your clipboard. You can also optimize each individual track by right clicking the desired tab (Track 1, Track 2, etc.) and clicking Optimize Track, but I'd recommend only optimizing when you actually copy a song into Mabi, and keeping the raw non-optimized MML for later editing.
I'm no musician so I'm terribly clueless with terminology, but I'll try to help out a bit. When composing in MML, you can set the lengths of your notes using l (lowercase L). Examples like l1, l2, l4, l8, l16, l32, and l64.
By default, note lengths are at l4, so you do not need to add a 4 after a note. For example at the beginning, r4e8f8g8>c8r8<b8r8g8r8g4, the beginning r4 and g4 have unneeded 4's, so removing those would freed up two characters. But because there's noticeably more 8 notes in there, let's use that same sequence but with L8. r4e8f8g8>c8r8<b8r8g8r8g4 -> l8r4efg>cr<brgrg4. Both of these will look the same in 3MLE, but less characters were used and that means more characters can be fit into a scroll.