Is it ineffective now?
Can you clarify exactly what you mean by that?
Are there side-to-side flex? Excessive slack before you get any tension on it? Does it not fire consistently when you pull it?
Well, there's a can of worms. If you just want a bit more than stock, I'd go with a motor like this: https://www.ak2m4.co.uk/internal-parts/motors-parts/gp-16tpa-motor-long running on a 7.4V lipo.
But many, many other options and opinions are available. You can go with a high speed motor, you can change the gears, but as above, once you open the gearbox, you can find yourself with no working gun pretty quickly.
Without opening the gearbox, you can fit a tighter bore barrel which might (hop rubber and air seal allowing) give you a boost. ZCI offer good value for money, although the range is limited just now because of shipping issues. Fortunately, AK2M4 has just got some back in stock, https://www.ak2m4.co.uk/internal-parts/inner-barrels?manufacturer_filter=15 and I believe (but check yourself!) that your stock barrel is 229mm. Now, you can go longer as you have a suppressor, but you won't necessarily get any more power or accuracy from it. Really, no joke, barrel length just isn't that important in airsoft. Anything over 200mm is good for getting 350fps - I just got 374fps from a 229mm ZCI barrel, an M100 spring and a good air seal, and had to go down to an M90 spring to get it down to 340fps.
I fit 50 or 60 degree Maple Leaf hop rubbers, but cheaper ones work as well.
The best thing you can do for reliability is to not open the gearbox.
If you do, I'd radius the front of the gearbox. You can also fit a sorbo pad, although I don't bother. Both are shown here:
Once you're in there, you'll want to check the piston air seal and lube and/or stretch and/or replace the piston o-ring: YouTube and Google will help with that.
You can also wire it for a mosfet while you're in there, which is a whole topic to itself.
The only real reasons to go in there for maintenance purposes though are: check and improve the air seal if your fps is way down and you can't sort it from outside the gearbox; or if you suspect that your trigger contacts are badly worn and need replaced.
Really though: if it ain't broke, don't fix it, or at least not until you have a working backup gun.
What I did on my MP5K was to fit a very small rubber o-ring onto the fire selector, to pull the other side tight against the casing and engage it in the detents. I'm not sure what size, maybe 3mm ID x 1mm, I just pulled it from a box of assorted o-rings.
The other thing that MP5s suffer from is sloppy hop units. I'm not sure if yours is set with rotating lever on top of it, or a foward-and-backwards slide. Either way, they're prone to moving:
If you have a lever with that issue, I'd shim it out to make it a tight interference fit, or (quite seriously) get it set then tape it in place.
If you have a slide-style adjuster, put a ruber O-ring around both it and the barrel, or cable tie it to the barrel, in order to hold it in place.
And I'll say it again for luck: motor, barrel, hop rubber first. I would not open the gearbox until you have a backup gun ready.