The most critical factor is the condition of the ionosphere on July 2, 1937, which can't be replicated.
It can't be replicated but it can, and has been, defined by Bob Brandenburg using ICEPAC. See
The Radio Riddle This explanation of ICEPAC is from
The Post-Loss Radio Signals : Technical Analysis ICEPAC[1] is the Ionospheric Communications Enhanced Profile Analysis and Circuit prediction program, developed by the Department of Commerce Institute for Telecommunications Science (ITS) at Boulder, Colorado. ICEPAC is a direct descendant of the Ionospheric Communications Analysis Program (IONCAP) and is a product of nearly 60 years of research, by ITS and its predecessors in the U.S. Department of Commerce, in collecting ionospheric data and developing methods for using those data in predicting and analyzing the performance of high-frequency (HF) communications systems which depend on ionospheric propagation. ICEPAC runs with the Windows 98 operating system.
ICEPAC computes HF system performance over a specified path, using a combination of user inputs and the contents of internal data base files. User inputs are: year, month, and day; the hours within the specified day, and the radio frequencies, for which performance results are desired; sunspot number [2]; the geographic coordinates of the transmitter and the receiver; the transmitting and receiving antennas to be used; the input power supplied to the transmitting antenna; and the level of man-made noise at the receiving site. The internal data base files contain: (1) statistical parameters of the time-dependent variations of the ionosphere [3] and of atmospheric noise; and (2) the characteristics of commonly used antennas in terms of antenna gain in one-degree increments of azimuth and elevation, relative to an isotropic [4] antenna.
For each run, ICEPAC gives the great circle azimuth and distance from the transmitter to the receiver, and the reciprocal azimuth from the receiver to the transmitter. Additionally, for each user-specified combination of time of interest and frequency, ICEPAC computes 22 parameter values, any subset of which can be selected for display in a variety of graphic and tabular formats. Signal-to-Noise Ratio (SNR) at the receiver site is the principal ICEPAC output used in this analysis. ICEPAC gives SNR in terms of the median [5], 10th-percentile [6], and 90th percentile [7] values, to quantify the range of statistical uncertainty due to random variations in the ionosphere and in atmospheric noise. Lucas and Haydon [8] provide a detailed mathematical treatment of the methods used in ICEPAC to model those variations.