Scooterotica

I can't believe I am having to ask for help on this but I am!
Having had scooters for 35 years, I can't imagine how many times I must have changed tyres on split rims - simple job & never ever had a problem.
Until now.....
I'm in the process of doing up an old Li125 S3 and I have bought a brand new set of Scootopia rims & new Michelin S83s.
Can I get them on? Can I hell!
I have tried everything - all manner of lubricants, heat gun to warm the tyres, standing on them etc but the rims just will not go onto the tyres.
They are ridiculously tight and I have had to give up before I either break something or hurt myself.
Any suggestions please?

Use tyre lube to ensure they correctly fit right to the edge of the rim. Other than that, just pump them up. That'll push the tyre fully on to the rim

I make sure I can get the valve for the inner tube through the hole in the wheel rim,and just sit the tyre on the two halves of the wheel rim,then fasten the nuts onto the bolts by hand.Pump up the tube just enough so when you tighten the nuts with a spanner the two halves of the wheel rim dont nip the tube.When tightened I pump the tube up until the tyre seats on the rim.

Thanks Guys. I eventually got them one using HxPaul's method.
After getting the valve through & one stud done up, I had to use a G clamp just to get the rims close enough together to get the studs through a bit.
Got nuts on them one at a time but with a bit of patience (not something I'm renowned for!) and I got them all tight enough to put some air in.
After a while the tyre started to slide onto the rims. Undid nuts one at a time, added washers and tightened them all up.
Pumped it up to 40psi and dropped it back again.
Never had such trouble - is it because tyres are made for tubeless rims now & have a stiffer bead or something?
Anyway, all sorted so back to the joblist. Cheers for your help.

Had the same problem fittin S1's to a mates. Did it eventually with lots of soap and once i got tube in an clear of rims i bolted them up and seated then with same pressure as you (40 psi). Think its as you said an due to them being made for tubless.

I recently fitted a brand new set of S83s to Scootopia rims with no issues. Used tyre lube, assembled with the tube very slightly inflated and once rims were bolted together I inflated fully which pushed tyres onto the rims.

A mate couldn't get a tyre on a Scootopia rim......