Coming back to the tire problem, it seems that the Kenda Kwest is very difficult to find and that there are no other tire in ETRTO 507x40 available (excepted some NOS of the ETRTO 507 Kojak).

Instead of choosing a narrow ETRTO 507 wheel size, it would have been better to choose the ETRTO 540 wheel size for which there are several relatively narrow tires available.