We do strongly suggest, given the nature of the games being created : Reboots from usually well known, and some times even existing franchises - that you include a disclaimer at your Game Entry at the Game Jam, explaining your motives and asking for consideration on behalf of the copyrights holders / franchise - so that they learn that the game is not meant to be freely distributed, published or sold.  That the games are just being made for fun, practice and learning exercise and that you'll remove it after the game jam's evaluation period ends. 

[ Please note that if you so desire - you can, when the game jam ends (after your entry get's judged / rated), use a different story / characters / art / music, etc.  Even add a few mechanics here and there, etc.  And you can totally transform your project into 'some other non-related to the reboot game'; in such case, that resulting new project you can totally publish (if not inflicting any copyrights then) .   So, while it can be seen for some as 'loosing your time', in regards of not being able to publish the game itself,  reality is:  You can still make full use of the source code you're developing.   Also, note that while you might not be able to keep the game jam's game posted along too much time, you can totally have it in a private portfolio website and can totally use it for showing, as part of your private portfolio material as well - so, in this senses you're not 'loosing your time' ]

Sample Simple Disclaimer Starts :   "... Please note that although we're recreating some of our favorite video games of all times - some of them are copyrighted material  - Like in this case, R-Type is a product of Irem Software - We're not affiliated with Irem Software and we're making this project just as a sharing and learning experience during the Game Jam.   It'll just be a replica of the first level of the original R-Type (1987) - but rebooted (refreshed visuals / effects / audio) - in this case by making use of Unity Game Engine.  It will extend to no more than that.  We do not commit to finishing it, as this is a just fur fun, practicing and learning event's project.   We'll not make the code or assets available, so please don't ask about it.  The prototype will only be available for the public to play it while the event is running, after this, the project will be held fully private and will only be used as game development portfolio material. ..." 

***Disclaimer of the disclaimer :   I'm not a lawyer *, so I can not warranty that it be 'acceptable' at all, you might be better of with one from the web maybe that can be formal and legally accepted - you have been warned - sorry for that, but its necessary yadda, yada.. mumble, jumble..