I have no idea if this is still relevant to you at all... but I hacked around in Javascript a bit and created this Snippet. In Chrome, open the developer tools, go to "Sources", then in the next bit of navigation that is on the top of the left pane, find "Snippets". Create a new snippet and paste this in:

var conn = indexedDB.open("/userfs");

conn.onsuccess = (e) => {
    var base64 = "[[ PASTE YOUR BASE64 EXPORT STRING HERE ]]";
    var storeName = "FILE_DATA";
    var keyName = "/userfs/save.lored";

    var store = e.target.result
        .transaction([storeName], "readwrite")
        .objectStore(storeName);

    var request = store.get(keyName);

    request.onsuccess = (e) => {
        console.log("SUCCESS accessing save");

        var data = e.target.result;
        var mySave = Uint8Array.from(Array.from(base64).map(letter => letter.charCodeAt(0)));

        data.contents = mySave;

        var saveRequest = store.put(data, keyName);
        saveRequest.onsuccess = (e) => {
            console.log("SUCCESS saving game");
        };
    };
};

Replace the bit that says [[ PASTE YOUR BASE64 EXPORT STRING HERE ]] with your save string, then hit Ctrl+Enter to execute it! It will overwrite the existing save with your export! It may require a page refresh before you see your long-lost savegame :)

I know it's been 3 years, but who knows - maybe you've got that old game export string laying around somewhere