Use the Denon and just add some mass to the tone arm
You can’t add 10-15g mass to the tonearm to make a low compliance cartridge like Denon 103 optimal for this lightweight modern tonearm. Even if you will add some mass you will have some problems with counterweight position, the counterweight is too light.
Also using a conical stylus in 2020 is anachronism, this cartridge designed in the 60’s for broadcast (radio). The life-span of the conical stylus is too short. It’s nearly 5 times shorter than MicroRidge for example. If you could use advanced MicroRidge stylus for 2500 hrs max, you can use conical only for 500 hrs max.
Rega tonearm designed for mid compliance cartridges. It’s much easier to find a better mid compliance cartridge (MM) with great stylus profile and extended frequency response than to mess around with low compliance Denon 103 from the 60’s on the wrong tonearm without detachable headshell.