If it is plain HTML, it is VERY EASY to convert it to a webtemplate as I mentioned above. Just copy/paste its source code to a new webtemplate in your Admin CP and add the [cantsee] tag I mentioned above, you are done.
Now call it as
http://www.yoursite.com/yourboard/sh...f_yourtemplate and here it is: If the visitor is a guest/unlogged member he will get please login/register to view this page message and if he is a logged-in user he will display your page. Very simple..