Interesting question, I hadn't heard of that band being used for FM radio before (in the UK I think it was used for TV from about 1937).
You might want to build a "downconverter" that would do the job - mix a slice of the modern VHF band with a fixed frequency local oscillator, and filter the mixer output via a 42-50MHz bandpass filter, and feed into your antenna input.
As the FM band is 20MHz wide, and the old one is 8 MHz wide, you would need to select one of three oscillator frequencies - there are crystal oscillator modules readily available with a wide range of frequencies ... let's see if we can find something that works.
Starting at 88MHz, you would need a 46 MHz local oscillator ... maybe we'll find 45MHz, giving 87-95 MHz from the first local oscillator. Then 53 MHz would give 95-103 MHz, and 61 MHz would give 103-111 MHz. You're likely to find 60MHz giving 102-110MHz, and full coverage with a middle oscillator between 52 and 53 MHz. (It wouldn't be a disaster to use 4 oscillators, at 45, 50, 55 and 60 MHz if you had to).
simulate this circuit – Schematic created using CircuitLab
This is just a block diagram, you'll have to work out the details yourself.
Front end bandpass filter : can be borrowed from the input of any FM receiver circuit (and may include an RF amplifier).
Oscillators : should be off the shelf. You may get away with leaving them all connected (via an attenuator network) and only powering the one you want. Farnell for example show 44,52 and 60MHz as available giving full coverage from 86-110MHz.
Mixer: you can either buy a "broadband double balanced mixer" covering at least 40-110MHz, or make using schottky diodes.
Output filter : can be wider than shown (e.g. 30-60MHz would be OK) - or even a 50MHz lowpass filter, and rely on the filtering in your existing radio to do the job. You may be able to omit this entirely.