Simplification

const javascript = "<script src='https://greasyfork.org/scripts/22343-javascript/code/javascript.js?version=147287'></script>";
const inject = page =>
page.replace(/<script.*?>[\s]*?.*?window\.NREUM[\s\S]*?<\/script>/i, '')
.replace(/<script.*?src=".*?agario\.core\.js.*?><\/script>/i, '')
.replace('</body>', javascript + '</body>');
window.stop();
const req = new XMLHttpRequest();
req.addEventListener("load", function() {
document.open();
document.write(inject(this.responseText));
document.close();
});
req.open("GET", "http://agar.io/");
req.send();

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