To answer your question, yes you must get a 63A sustained rated to replace a 63A existing component.
Don't cheap out to get a 40A sustained  rated part.

300mA tripping threshold is elcheapo protection for fire only.
Human already die when >50mA pass thru human body.
300mA tripping threshold is 6 times con-con-con-con-con-confirm die.
Or, rather, closing barn door after the horse left.

I put nothing more than 30mA tolerance RCDs for the places I can control.
Because that's the way. In SG and AU all use 30mA for domestic. In MY life is cheap.

Well, same issue I faced..

They insisted water heater uses 30mA and when I said ST rule since 1994 is also there already he looked at me and insisted "no such thing ever!"

If you still worry of easily trip (despite the good brands like ABB, Hager won't unless it is a life-threatening leak) then you can continue with 100mA for your lighting and fan, since human are usually not exposed to it. Keep the 30mA maximum for your plug points, and 10mA for water heater.

Bear in mind, you should in fact ensure your steel roof/steel-type awnings are grounded as well (a green wire to connect to your DB busbar directly) for safety - otherwise ((when there is a leak) anyone who climbs the roof to fix/repair something might get electrocuted to death since your lighting/fan RCCB sensitivity is only at 100mA. This also ST requirement actually.
It would be a miracle if people who install awning at his house would get an electrician to help pull a green wire connect to your earth busbar, whilst the awning are fitted with wired lamps/fans.

It doesn't trip often if you buy a genuine ABB or Hager 30mA, current models.
Yes 30 years back models even 300mA also trip every other rain lah. The technology then and now improved a lot.

Nah I show you lah, I moved to SG and left my apartment fridge running.
Its protected with whole house 30mA, ABB.
[attachmentid=11222407]
100mA for lights and fans?
You still use flourescent tube or halogen high-bay lamps at home??
seriously, 100mA?

Buy a good branded (read: ABB or Hager) 30mA whole house je lah. Dont waste money buying 100mA.

Yes but I think the issue we have in Malaysia is enforcement... You can see many electrical shops also selling DB box with 300mA rccb for domestic use. Wiremen taking contracts are mostly non-licensed whilst licensed ones simply charge with no control.

My house wireman luckily is really a knowledgeable person when I asked him to get ready earth wire connected to busbar for my future awning - he said "Singapore standard is like that" - by Singapore standard it does not mean just the guideline but also the enforcement!

I don't have his contact, but only my main con's contact so you might not be getting the 1st hand price.... I am also waiting for the chance to get his

Better get plumbers who know how to install storage water heaters to do it for u. At the inlet, u will need to connect it to the pressure relief valve too (water source piping > pressure relief valve > isolation barrier > storage water heater's inlet).

Yes it looks like the one in your picture, but better purchase from the storage water heater's brand distributor so that u get more assurance that it is a confirm fit.

joven storage heater installation sample

https://images.app.goo.gl/h6zCNif8q7vJGr8b8

https://joven-electric.com/my/product/stora...my/jsh-my/jsh35

Suruhanjaya Tenaga recommends Isolation Barriers to be installed at both inlet and outlet. It is always better to be safe than sorry.