It's probably best to tinker with PICs so that among other things (the thrill of making stuff happen) you may have some idea of what to do if something goes wrong. Now I realize that writing the code for this yourself is a bit far fetched at the moment.
It seems to me that a 13Mhz crystal was either something he had on hand and needed to get rid of or some tight code loop needs exactly the right crystal frequency. Using LEDs does place some overhead on the task, as does serial communication.
Were I to make one, which some day I hope to, I would used a timed interrupt every millisecond to read TMR1 and figure how far it has advanced since the last iteration. This will allow up to 65MHz, which is beyond the rating of TMR1's input of 50MHz. The external chip is a high speed divider, which is what I would use to get the high MHz or GHz range.
That much having been said, I recommend looking around for another project like this one. I believe you'll be able to find one that uses most of the same parts, except for a more common crystal.
After finding the above mentioned article independently when running another search, I thought about it more. This is the first one I've noticed that has an RF probe built in- wonderful if you're a radio enthusiast but may complicate using it some times.