Heeeeelllppp Meeee!

Honestly...

A 300 2 stroke is almost always the answer.

I voted 125 just because of the fun, they're both kinda,  different ends of the spectrum. you'll have to work harder to keep the 125 running a decent pace, where the 450 is honestly too much for most offroad.

Get a kdx200 until you get your riding fitness back. A 450 will kill you and you won't be able to hold onto the 125 for longer than 5min

350xcf or 300xc, just decide on if you want 2 or 4 stroke and don't over think it. 350 is plenty of power for nearly everyone. 

Wow, an off-road 125X, or a KX450, that's quite a difference in power!

I say you should check into a 350; see the link below... 

https://www.vitalmx.com/forums/Moto-Related,20/Should-of-got-a-350-year…

You would definitely like joining the 350 cult

If those are your two options, it's 100% the 125. You'll have fun on it. 450 after a lot of time off will take a long time before it becomes fun again. 

I had a squaomos cell tongue tumor in 2020 at the age of 60. Mine was too large to remove and had spread to a lymph node in my neck. Lots of radiation and chemo, and at the moment I’m doing OK. Hang in there.

    Hoping for the absolute best for you Buckeye . As far as the bikes go, my opinion is the YZed 125.  Strength may be a factor, and the 125 will be easier to handle, load, and work on. You can see how it goes, and buy anything you desire after you get past this !

A 450 motocross bike sounds like a terrible choice for what you want. Why not a kx250? That would be more suitable and I’d think it could be had cheaper than the 450. Given the bikes you mentioned, the 125x for sure. 

I've ridden a 450 and 125 on the trails and hated them both. Yz250 2 stroke is the best I've ridden on the trails. Light and snappy and the extra power over the 125 makes wheeling over logs and holes much easier

KX450F

CR500. The answer is always a CR500.

After around 10 years off I started back with a CRF250X, I was lots of Fun. Good Vibes sent your way.

Yeah I'm thinking a KX500 maybe. 

 "Dirty Trail Action".

So, I'll approach it as that is what you (mainly) intend to do.

KTM200, as late as you can get, but, bloody hard to get, as people are keeping them.

Beta 200 - for a brand new bike, or, a couple of year old bike.

Both are pretty much a 'Big' 125. 150s are that, but 200s give you just that bit more 'usability'. 

300s you regard as 'megabux' ? There's a zillion of them around as 2nd hand, and, if new, well, they are no more 'megabux' than most other bikes.

The various 250s that the 300s are just a bigger bored engine (though, later Betas are, I think, a different bore And stroke to their 250s) and are great things, and, there are plenty around, and, generally can be had for better prices than a comparable 300.

The difference between a 450 and 125 - it's a whole bloody Universe, that.

It can be done. I've never had a problem with going from one extreme to another . It's so marked / extreme that You  (well, I) never mix up what you are one. I've got myself into more trouble on 250 /300 2Ts when going back from my 500 - they tend to feel like a Toy (so Light) to me, when compared to my armoured up Ancient Teddy Bear of a 500 2t , but their power is anything but toy like.

Grab the kx450 and throw a Rekluse in it. 

450 is still my bike of choice overall, but i find it harder to ride on tight single track then a two stroke. For me, the 125 is just too underpowered unfortunately for trails. Granted, my 450 would be much better if I actually set it up for trail riding but I prefer to keep my mx settings. May try out a 250 two stroke as well. 

ive got a 2021 te300i for 6k. just throwing that out there. I wish you well with your health, and i truly hope you have many more healthy years of riding!!

I hope all goes well for you! 

A buddy of mine wanted to get back into trail riding and he was hung up on buying a 450 against my advice.

I actually begged him not to make "that mistake "  and he bought  a KX 450 anyway.

He struggled terribly as the weight, stiff suspension and gearing is not suitable for trail riding.

After it over heated in the woods I let him ride my 300 and he sold his bike right away.

Don't be hung up on buying whatever your dealer has laying around but rather the right tool for the job.

MX bikes are not the right tool for trail riding.

Almost anything between those 2.  If it has to be one of them, then i guess the 125.  Id think any trail specific 250f-300f would be a much better match.  

I had squamous cell cancer on my scalp that spread to lymph nodes in my neck despite surgery and radiation. I had surgery on my neck, which unfortunately required them using my pectoral muscle to fill the hole they left. But the immunotherapy I'm on is doing its job so far and maybe my KTM 150 will come out to play next year. I'd hate to think of riding anything heavier.