Whether you decide to use UnleashX as a main dash or not you should always have an alternative dash or dashes installed in Apps. With UnleashX it is particularly easy because all you need is the XBE as, on launch, it will generate its own default Config.xml and Item.xml.
Just put the UnleashX XBE in a separate named folder ("UnleashX") in E:\Apps or wherever your Apps folder is located and, if not already named "default.xbe", change it to that using the XBMC file manager. Reboot and it should appear in your XBMC > Programs > Apps folder.
You have a ready source for UnleashX dash(es) if you used SID :-
Either look in the original unzipped SID Installer > UDATA > 21585554 > 000000000000 folder for "menu.xbe" or on the softmodded Xbox itself in your E:\UDATA folder using the XBMC file manager.
That is actually an UnleashX launcher, typically UnleashX 0.39.0222A Build 572, although in later SID versions that might have been changed. As described earlier copy it to a temporary folder simply and change the name from "menu.xbe" to "default.xbe" and do what I said.
There are loads of others places you can still find various UnleashX dash versions but the final version: v584 is not considered the best. It has issues in a number of areas so look for the earlier v572 if the menu.xbe you have turns out not to be v572.
Whatever your main dash I'd recommend installing all the main alternative dashes too (XBMC, UnleashX, EvoX, Avalaunch) in Apps as they both have unique tools that may be of use to you. It is also useful with any main dash to have a testbed copy installed in Apps too. XBMC for instance can be rendered unbootable by using certain older skins so apply any changes you might want to the testbed copy first to save you from that pain.
If you didn't use SID's Virtual C + Eeprom protection options I'd also suggest updating your softmod using Rocky5's 2016 Softmod. It is currently the only softmod package being regularly updated and has features which make it, arguably, the safest softmod available eg. unlike SID/AID most of the important files are located on the protected C:\drive.