If you have removed a page part from a page in your site, chances are it's just uninstalled and not irreversibly deleted. Unless you have selected Yes, irreversibly delete instead SiteWrench defaults to simply uninstalling the page part. Uninstalled page parts live in the Page Parts system in SiteWrench.
Here we can see a list of all the page parts you have access to, determined by the hosting plan of the site. For this example, we'll bring back a Photo Gallery that was uninstalled. Scroll down to the Photo Gallery page part in the list. From here, we can see all the instances of the Photo Gallery page part across our site. Right now, there's just one.
Under the name of our Photo Gallery, there are three icons. Select the Install icon (the loop back arrow). From this modal, we can select the page we want to re-install our Photo Gallery to. This process works for every page part in SiteWrench.
NOTE: This will NOT work if you have irreversibly deleted the page part. This only works if you have UNINSTALLED the page part.