Memorize music from the scores. This is a great time to develop this ability. I've tried it, and it is doable, but it takes longer than memorizing with a piano for me, yet, because I haven't practiced it enough. Can you write? If you can, work out fingerings for your pieces. Harmonically analyse your pieces and understand how they "work." But if writing isn't a good idea, which it probably isn't, then listen to recordings. Practice singing your melody lines, or separate voices in any fugues. Do memory points. (Numbering every other or every few bars - every other works best - and then drawing those numbers at random and visualizing in your mind, what the score looks like at those points, so you could start at any one of those if you ever had a memory slip. My teacher just told me last week, and I really think I believe her, that if she had 2 months to learn a piece, she would not panick if she did not have access to an actual piano for the first month.