There is no way to make a backwards facing OHV engine look vintage. I don't care how much stuff you bolt on, it will always be facing backwards, and anyone who notices you have a vintage bike in the first place, will notice you have a Chinese engine on it.
So just roll with it. Nothing wrong with making a machine go. Of course you will be a traitor to the sport, and guys like me with a purist attitude will laugh at you, as you pass us by at half throttle.
But trying to fake it is worse than using the engine in the first place. LOL.
There was a member here a few years ago, did a green Cat 250 with a Predator on it, and mounted the engine backwards, so it faced forward. He then fabricated a gear box to run the drive backwards. Last time he posted, he was having some trouble with it, as they were helical cut gears and the entire affair was complicated. But the engine looked good like that.
Note that you can make these flat heads run much better. Many of us have done that, Briggs is the easiest, since there are a lot of aftermarket parts available thanks to karting. Also, some of the guys on here have made some very strong big blocks, but they are more knowledgeable than I. Ole4 and Delray to name a couple.