As with old sets and systems (Power Functions (PF) in this case) are being retired supplementary sets become discontinued as well. There are couple of ways you could handle this situation.
In case you have a set which suggest using 8293 set for motorisation it is advised to source that particular set or elements of that set. While LEGO no longer sell 8293 set directly it is still possible to acquire this as set as New or Used on secondary market, like BrickLink, BrickOwl, Ebay, etc.
Depending on your LEGO collection you may already have some of the individual parts included in 8293 set since they are not unique and have been available in other sets too. Here you can to buy just the parts you need.
LEGO has decided to discontinue PF system to make a way for newer one called Powered UP. Sadly, at this point of time new system doesn't have have a similar supplemental set to motorise sets. This means required parts need to be sourced individually.
Another issue with PU system - it is not 1:1 copy of old system in terms of element dimensions and properties. But let's review contents of 8293 set and see what PU system could offer.
8293 set contains 4 items of PF system (below) used to bring models alive. I'm going to dismiss non-PF items as these would need to be acquired individually anyway if PU parts are going to be used.
- Battery Box. PF system use standalone battery box, where power to motors is applied directly by using switch on a box itself compared to 88009 PU Hub (LEGO, Bricklink) (which is both Battery Box and Receiver) which will only provide power once signaled from 88010 Controller (LEGO, Bricklink) or smart device via Bluetooth. Next thing is the different form-factor - PU Hub comes in different shape and size (similar to PF small Battery Box), which means it may not fit the originally intended place within a model designed for 8293 set.
- M-motor. Luckily, new 45303 M-Motor (LEGO, Bricklink) is very similar to previously used PF M-motor. Not the same, but most won't be an issue.
- Pole Reverser / Polarity Switch. This item does not exist as PU element.
- Lights. Similarly to PF Lights, newer 88005 PU Lights (LEGO, Bricklink) should work fine.
As seen above, newer elements have quite a different properties and may not fit sets originally designed for 8293 motorisation without substantial modifications or currently offer nothing as a replacement (like a Pole Reverser / Polarity Switch).
It is also worth mentioning that PU system has new connector, so none of the previously released motors/electric components incorporated into new system.