Just finished a ROM editor for Sonic Spinball. Will work on any version and any region except for the very first prototype (Aug 1993).

There is some detailed info (below) about how to beat Robotnik (level 4 boss), as well as some possibly undocumented info on some of the prototypes.

Lots of fun stuff to adjust, like the amount of gravity, how much control you have over Sonic while he's curled up in a ball ("steering" him), enable a warp area in level 1, reversing gravity while playing, etc.

Can get it from my site: https://codehut.gshi.org/Download.htm#SpinballEditor
or from RHDN with a full description: https://www.romhacking.net/utilities/1690/

Robotnik (level 4 boss) was always the most difficult boss for me, so I spent a lot of time making a few options to make it a bit easier to defeat him. You can also make it harder to beat him if you'd like.

Since some of the information on the internet on how to defeat Robotnik is either incomplete or incorrect, I thought I'd add a bit of info I found while working on this...

Level 4 boss info (Robotnik): Once you collect all the emeralds, you will have access to the boss room (space ship). Once inside, you will find a button under Robotnik that you can easily hit using the flippers. Some guides say to hit this button 5 times to give you access to Robotnik, but actually, you need to hit it 3 times, and only 3 times. Hitting it a 4th time will re-block access to Robotnik.

* After the first hit of that button, the left robot arm blocking your way up top will disappear.
* The second hit of the button will make the right robot arm disappear.
* The third hit of the button will make both of the socks disappear (they also prevent you from getting to the top).
* Once you hit the button for the third time, you'll have a clear path to get above Robotnik and start attacking his shield. It takes 10 hits to destroy his shield.
* After you hit the button for the third time, it moves a value of 0x05DC (hex) to a timer (which equals 30 seconds). During this time, you will have access to Robotnik. Once the timer reaches a value of 0x01F4 (hex)(10 seconds), it will display a warning message about you running out of time. Once the timer reaches zero, the socks will reappear and block your access to Robotnik. The editor lets you adjust how much time you get.
* If you accidentally hit that button a 4th time, you then have to hit it several more times (I think it's 5) to re-open access to Robotnik. Same thing if the timer runs out.
* Also, the timer stops counting down whenever you're near the top of the ship, and then continues counting down when you get towards the bottom.
* One other note, if you try to go to the top and one of the socks catches you, it will throw you against a window. On the third hit of the same window, it will break open and you will be throw out of the spaceship. The editor has an option to prevent the windows from breaking.

The editor offers a new way of getting access to Robotik. Every time you hit one of the robotic arms or socks that are blocking your access to Robitnik, that robotic arm or sock will no longer block your way. The specific arm or sock you hit will still be visible, but it's collision detection will be turned off and you will pass right through it. If you enable this option, you can get up to the top of the ship and start attacking Robotnik with just two hits on either side (one hit for the arm, and one hit for the sock). You don't even have to hit the button if you don't want to, although you can still use that method if you'd like.

Some info about the early prototype ROMs:

There is a Sept 17, 1993 prototype ROM (TCRF has info on it). The level select button cheat that's everywhere on the internet (for the final release ROM) is different than this Sept 17 prototype:

In the Option menu, press: D, A, D, B, D, C, U, A, B, U, A, C, U, B, C

If entered correctly, you will hear a chime. The way you select a level is the same as the regular ROM (which is all over the internet). TCRF says that the level select cheat doesn't work on an early version of the game, so I'm pretty sure this is the one they're talking about.

There is an even earlier prototype (August 1993) that has the level select button cheat enabled by default. However, the buttons you press to start on different levels is a bit different than the regular ROM:

At the title screen, hold A and press Start to start on level 4. Hold B and press Start to start on level 2. Hold B + C and press start to start on level 3.