Building a 4-stroke bicycle

If you don't have a calibration gauge a simple trick is to use an adjustable wrench. Just adjust the wrench to fit the bottom of the shaft tightly. Put a piece of tape across both sides the wrench adjuster to hold it in place. Then slide the wrench up the shaft. If it stays a tight fit then it's a straight shaft. If the fit gets very loose then it's a tappered shaft.
Thanks for this, you need to remove the transmission access the shaft right?
So I ended up get the grubee transmission:

So I ended up get the grubee transmission:

View attachment 211203
Great transmission. Are you going for a single ratio range or do you want a shifter bike?

I'm assuming 5\1 transmission and 26" tires. For a single ratio to do 30 mph on level ground use a 14t\48t

If the wheel size is different and/or the transmission is different let me know what they are.

I've never liked a 9t sprocket; I don't like to drop below a 10t sprocket. It's like asking fewer men to distribute the same load; it causes the teeth to be prone to hooking, bending, breaking, and rounding off. It also means your roller chain has to condense down to a smaller circumference putting more stress on the pins and side plates.
Honestly at this point I'm not sure. I know this transmission is 100T/20T, he was out of the other one.

I was thinking of getting a 36T wheel sprocket, but again I don't know jack s**t about the drive ratio stuff.

Yes 26 inch tires.
This is the same ratio that I am running with a 44T on the wheel. If you really must have a 80T belt pulley check with Molly at Bicycle Engines. She might have one. There's a 44T sprocket and rag joint in our FOR SALE forum.
I'm getting one of those sprocket hub adapters. I like those more.

I thought I read in this thread that you could change around the output sprocket and wheel sprocket to achieve a certain ratio instead of buying a specific 80T/20T vs 100T/20T?
I'm sure I posted a gear ratio / speed chart for you somewhere in your thread. If you can't find it I'll do it again. You'll first need to identify which of the many wheel sprockets have the bolt pattern for the adapter you intend to use. With this list you can choose a small drive sprocket to obtain the speed you're looking for. One thing to remember is not to solely rely on the "clamping " feature that adapter has around the hub. Many of us would want to install some kind of a pin or woodruff key. This you will need to engineer yourself. Your bike is looking good so far and I'm sure you'll end up with something to be proud of.
So with the 100T/20T trans, and a 10T output sprocket with let's plugin the 36T wheel sprocket.

What would I be getting out of that setup?