Where can I find the regulations on the minimum stall speed for a GA aircraft?
On August 30, 2017, a completely new body of regulations for small aircraft has been put in place which does not distinguish between normal or utility airplanes but low speed (below 250 KTAS) and high speed designs (> 250 KTAS). It also distinguishes between four levels depending on the number of seats (0-1, 2-6, 7-9 and 10-19 passengers). The spirit of the new regulation is to no longer prescribe design-specific rules but more abstract goals in order to limit risk, leaving the details to the aircraft builder.

Also, the number of rules has been drastically cut, and their numbering starts where the old numbering ended. Therefore, 23.49 has become 23.2110 and no longer gives a specific stall speed. All it does is to direct the applicant to determine a stall speed and details what conditions must be covered.

I expect that you have not formally started the certification process of your design, because that date determines which regulations apply. You are now free to select the stall speed of your design as long as you can demonstrate that the aircraft can be operated safely without exceptional pilot skill or strength.

But you can always count on Eurocrats when a body of regulations which has grown like cancer is to be preserved and extended. In their version (CS-23), the maximum stall speed of 113 km/h or 61 knots is still alive and well. However, Amendment 5 has now copied the new body of regulations as well and replaces the old certification standard in the name of global harmonization.

The old regulations reflect the experience which has been collected over decades. Many rules can be traced back to a specific accident and it helps to read through them and to consider why this specific rule has been implemented. Your new design might employ a clever way to avoid that particular condition at all, and then the new liberty of the new regulations will be helpful. But it would be foolish to throw all that experience overboard - a lower stall speed makes any airplane easier to fly. When things happen less quickly and less kinetic energy is involved, more accidents can be avoided and the damage in those that still occur will be lower.