Can I Let's Play Cave Story?

You most certainly can, according to Nicalis   who holds commercial rights to the game. This also allows ad revenue from said videos.

How do you convert .org files to .mp3?

There is no direct .org to .mp3 converter. There are reportedly three ways to get around this (in descending order of faithfullness to the original):

The first is to record directly through your sound card as it plays. Personally I would recommend a program like Audacity to do this. However you need to make sure that stereo mix is enabled (Realtek tends to disable stereo mix by default), plus the recording volume needs to be tweaked until the recorded music and original sound like they are at the same volume (this is so that high notes don't get truncated). Keep in mind that your playback volume also influences the volume of your recording, so be consistent. Due to this level of complexity this can only be recommended for advanced users.

The second is to convert the .org file to .xm using org2xm, and then use an .xm to .mp3 converter from there.

The third is to use the org to ptcop converter to convert to a .ptcop file, use pxtone Collage to export to .wav, and then finally use a .wav to .mp3 converter to finish the conversion.

I'm translating the game to my language, would you put it on the site?

Of course. Email me a link when you are finished.

Do you have any advertising space available on this site? What about pre-written articles and/or link exchanges?

All content (links, text or images) on this site must be to do with either Cave Story, Daisuke Amaya (Pixel), or things that he has had involvement in creating. I refuse to sell out in any way.

Would you like us to improve your SEO?

No. The site has sat within the top 3 search results for "Cave Story", "Doukutsu", and "Doukutsu Monogatari" for over a decade and an half.