They are all 'traditional' mudguards. You can swap the 350 mudguard/fender on your B5 with a 500 'round' style one. The connecting points are the same, and it will require minimal alterations, if any. The solo seat can be installed on those round style 500 mudguards. Again, a little DIY might be required to get everything installed flush. But that's the charm of working on these bikes.
When you mention 'vintage style exhaust', are you referring to the same old short bottle type silencer (This has been done by various members on the forum)? Or the whole 'C' shape header pipe + silencer combo? This can also be done by fabricating the 'C' shape header pipe, but it will be a tough job because of the oxygen sensor.
The wheel size is the same old 3.25x19'' at the front, and 3.50x19'' at the rear since the past 65 + or so years. The new domestic market Bullet 500 has an 18'' rear tyre these days. Same has been done on some forest green B5s I think. Not sure why they've done that. The UCEngine is kind of smaller than the cast iron one in terms of overall 'length'. To compensate for the smaller UCE, the swing-arm on the Bullet standard/B5 has been extended.