A cartridge version? How much is that? And from where? And how would you update a cartridge version with bug fixes?
The game itself is free (we don't sell cartridges or disks, we're not making any money on this game) and the .crt image is included in the download, if you want to use the cartridge version you can either:
1) use the .crt file and mount it in a C64 emulator like Vice - unlike the disk images, loading will be instant, so if you're playing in an emulator I would use the cartridge version. You can find Vice for many different platforms here:
http://vice-emu.sourceforge.net/ - after Vice has started, select File -> Attach cartridge image -> CRT image, and then select the Ultima .crt image in the file requester.
2) flash the .crt image on an EasyFlash cartridge or any other piece of C64 hardware that supports the EasyFlash cartridge format. These cartridges can be reflashed on the Commodore 64 with updated versions of the games (or new games when you tire of the old ones). The latest version (the Easy Flash 3) has room for 7 EasyFlash cartridge images - it supports other cartridge formats as well, and has a few additional features. The Easy Flash 3 will set you back ~$65 fully assembled while older versions can be found fully assembled on ebay or elsewhere for around $36, even cheaper if you buy a build-it-yourself kit (~$25) or just the PCB+case (~$13). You would also need a way to transfer the file to your C64 for flashing (usually onto an SD card using an SD2IEC or similar solution for the C64, or a 3,5" 1581 floppy disk).
A few old games have been fixed for the EasyFlash, and at least one was specifically written for it, the C64 version of Prince of Persia:
https://www.youtube.com/watch?v=mdwHe4lO8as - it can be downloaded for free from
http://csdb.dk/release/?id=102540 if you want to give it a try in the emulator. Commando Arcade, a completely rewritten version of Commando also has an EasyFlash version:
http://csdb.dk/release/?id=137173 - if you search for easyflash on csdb.dk you will find a bunch of fixed games and compilations of games for the EF.