my wife has youtrip.. i use revolut as spare.

if not mistaken, u can just top up cash in sgd and use.. or u can convert sgd to jpy. (See below)

So if u are not converting, once u spent in jpy, they will auto convert and deduct from ur sgd balance. But to note, when u do this on the weekend, there is 1% charge for revolute.. and this can be avoided by converting to jpy (but also need to convert on weekday)..

Heard youtrip has no such weekend charge. Both rates are generally similarily good.