i am trying to buy a new crank arm however there are 2 different type, a rhombic hole or square. After looking at the picture on Amazon, they look both the same. any help would be welcome.
Look at the square hole at the root end of the crank. You'll see that the square hole type has a square where two of the sides are kind of parallel with the crank.
Looking at the rhombic hole, the pictures will show that whilst the square hole is still square, it is turned 45 degrees and you could say it is more like a diamond, the edges of the hole are pivoted.
The holes will be the same, just turned by 45 degrees so it's important to match the existing good crank so they end up aligned. So if you are replacing just one crank, look at the square and see which way it is lying and match up accordingly.
Nearest I can find is https://www.bhfitnessusa.com/manuals/final/sb2.pdf which doesn't say anything about the BB/crank interface.
This picture also doesn't show the crank end, but based on the overall design, I'd be 70% confident its a square taper.
To be sure, lever the small round black plastic cap off one side. It is unlikely to be a screw so don't use a screwdriver in the slot.
There will be a hex-head bolt or nut visible, probably 14mm. You may need to remove this with a socket driver to see around it.
This is a square-taper in position on a regular bike:
If you see something different then its one of several other standards:
The tool to remove the crank arm from the axle is called a Crank Puller and look something like this:
Then the new crank arm is drawn up the taper using the pressure from the bolt/nut.
One significant gotcha is the new crank - a conventional bicycle has a thin left crank for a left pedal on the left side of the bike, and the right side crank has a spider for interfacing to the chainrings, and is tapped for the right hand pedal. The PDF manual above states that this exerbike uses normal bicycle pedals, with left/right mirror.
So if you get a single bare crank, it might be for the left side, and not be able to take your right hand pedal if used on the other side. If you use two left pedals, the flipped one on the right could undo itself.
If you get a right crank, then it may have enough lippy bits of metal in the way to interfere with other items on the bike.
Good luck! This is achievable, just watch out for the pitfalls.
If you can't find one crank exactly what you need, then you might need to buy a pair.