Technically you could port something by reverse engineering it... but that's difficult and raises copyright concerns.
And technically you could sell an open source engine which was ported to Dreamcast, just not the music, sound effects, levels, images, videos, etc. that are the data assets which exist outside of the open-sourced engine. But when people buy games, they are paying for the ability to run the game and the convenience of running it without technical steps like combining a scrambled binary with a commercial game's data and compiling that into a CD image that can be burnt at home.
As to Arcade Racing Legend, I am intrigued, just as I was intrigued by 4x4 Jam's Dreamcast port. However, I am not willing to pay $40 for a port of 4x4 Jam, a game that costs $7 on PSP and is free-to-play ($0) on Android and iOS. Even the in-app purchase(s) for Android and iOS only run from $1 - $4, so let's call either port a $4 purchase. Considering these, I'd be willing to pay $15 - $25 USD for 4x4 Jam, no more.
For Arcade Racing Legend, I'd say $20 - $30 seems fair to me. I can't say whether that's "really fair" or not, but personally I can't justify paying more than that considering all of the affordable professional and indie commercial Dreamcast games available as well as the free homebrew releases. I encourage this release and others like it to continue to be made. However, just like the "expensive European indie Dreamcast games," I am torn over the price. If I were to buy Arcade Racing Legend, then I'd probably wait until it was available for shipping instead of pre-ordering it. While JoshProd has been reliable in developing and shipping what he promotes, all of these other "infinitely delayed," "radio silenced," or outright cancelled Dreamcast indie releases have me gun-shy about paying in advance for their release(s).