There is no way I would mess with factory Crossover of New Speaker, UNLESS many people documented the OEM made a mistake and problems resulted.
The crossover looks like simple design nice quality to me
https://audiogroupdenmark.com/product/boerresen-x3/
OLD Speakers, I would have my friend check the crossover for out of spec parts, replace them with the idea or restoring 'original', not 'let's upgrade them".
He has two pairs of JSE Model IIs. restored one pair's few out of spec parts. 'Upgraded the heck out of the other pair. We both prefer the simply restored pair to the upgraded pair.
It's very hard to deny the bias of it's better right! he's like me: not necessarily so.
Vintage parts might have been +/- 5%; newer ones, same values are probably tighter specs, say +/- 2%, not going to improve the sound, just typical modern production.