Some info on hull type and length, brand of boat. Type of use boat has.
How many people you carry , how much gear.
All relevant info to provide better replies.

Mclay 650 cruiser, generally 2-4 people for day fishing, Once or twice a year will take 5-6, Overnighters and longer trips 2 people but a bit more gear, food ice etc. I had been carrying 30 liters of extra fuel but with 4 stroke economy I can ditch that. Cheers

In my opinion a 250 Yamaha would be a big motor on a 6.5. Perhaps you should wait.

Contact Mclay and ask them maybe? Margins are often built in so ,may still be in spec. Worth a shot mate

The problem is not always the hp, it's the deadweight on the stern, and if the boat is a bit light up front, the bum sinks and the boat gets all tippy and unstable. Gets worse when most of the people on board rush down the back to fish or look at something. The problem is even worse on a pod, as there is a lack of buoyancy right under the motor.

Dealers have stashed motors for their own builds..

Muzza-Jay wrote: ... I’m not sure how the insurance works with this type of thing? ...

Muzza-Jay wrote: Thanks for the feedback, i thought my trim tabs could help with the buoyancy or potential porpoising, I think I’m best to wait and go for the correct weighted engine. Cheers

Trim tabs only help once you are under way..
Really though, another 20 kg?, it's not a lot percentage wise, but I am aware of the negative effects of an overweight motor can cause.

Thanks for all the advice, going to wait and see how things pan out later this year with engine imports, hopefully this bloody COVID thing settles and allows us the freedom we all need to get out on the water. Cheers

Comments on the extra 15/ 20 kg above being ok
Totally agree.
 As to a bit over powered, only if you use it it.
 May sit at anchor 1/2" lower if anything in the water at anchor.. doesnt matter.
Will cruise more economically (10%+) at the old engine cruise speed due to less throttle , ans less load on engine meaning far leaner mixtures.
Will cruise more comfortable than before, faster and use maybe little less to same fuel as before.
 Hit chop, can set throttle and cruise thru more comfortable, slightly faster, and on an oily rag, as dont need to work the throttle.
Crossing a bar, wave stand up , have the power to get to it and thru the next wave before it fully stands up.

Down side.. must look around, ask and check everyone is holding on even on a normal hole shot.

As to trim tabs etc, will not need them because of the weight (only 15/20kg) .. you have all that extra reserve power, very little leverage required to drop the bow... And the choice of prop rake will sort that out...

Get it..

I'd agree if it was a 300hp he was looking to put on, but an extra 23kg? Move a battery forward if you must, or don't use the livewell. I'm not advocating that he do it, I'd be happy with the existing motor if reliable, but I really don't see this making the boat unsafe. It's the same as a child on a stern seat.