Honda SS50 1977

[TonUp]

I am restoring a Honda SS50 1977 and I need some help!

First question, what are the correct wheel sizes for this bike?
I have a front wheel with 2.50 - 18 tyre (no markings on the rim) and on the rear I have 2.50-17 tyre on a 17x1.4 rim.
At a guess, the front rim should be a 17x1.4 same as the rear?
The front rim is pitted and the spokes could do with replacing anyway.

Next question, the bike currently has a Honda cub 90cc engine (HA02E - electric start, 12v) fitted.
I have the Honda 50cc engine as was originally fitted to the bike.
The 90 engine has a neutral, down 1st, down 2nd, down 3rd, down neutral rotary gearbox.
I am assuming the the 50 has a neutral, down 1st, up 2nd, up 3rd, up 4th gearbox.
Can I swop the gearbox from the 50 into the 90 engine?

I have a friend that has worked on a few of these cub engines and is happy to tackle the gearbox. Just need some pointers as to what is possible.

Thanks for any help!

[totty]

The 50 could be a 3 speed. A lot of parts can be swapped on these engines but I don't know this specific combo, 3 speed casings need machining for 4 speed boxes.
What result are you after, the 90 order but without the rotary? So you'd need the 90 shift drum and 50 detent.

There's some good experts on these motors on the monkey bike sites but the main uk forum is very quiet these days. Shire Bikes will more than likely know, they'd also be able to sell you a 5 speed box, big valve head, long stroke crank and bore up to 106cc so you can stop worrying about being hit from behind.