Sistering vs cross bracing joists in attic

[Archiethere]

Hello I’m about to start a project on my loft. I’d like to use it as storage, not a habitable space. When I’ve cleaned up half of the space the joists aren’t the best (approx 75mmx33mm) so I would like to strengthen them. There are old lath and plaster ceilings below these. It’s my understanding that you can either sister wood to the existing joists or run wood parallel to them in order to strengthen them. Which one would be best?

https://imgur.com/gallery/qMifKUv

I’ve included a picture of the half of the loft that I’ve cleaned ready for work. I have purchased a lot of 4x2’s (c16) for this task but I’m wondering if they are too heavy? There’s a supporting all about half way through the attic where you can see they have joined the joists on the photo so I was thinking if I did the same it may be ok? Any help would be greatly appreciated thanks

 

[blup]

Can't make out much from the pics, but 4 x 2 will not be too heavy for the supporting wall and the outer walls if all are structural. Obviously they will be higher than 3 x2's, so boarding out will need to take this into account.

Blup