LS swap is "me too" as well, just my opinion. Briggs 5hp were used in untold number of racing classes so pretty sure there is aftermarket support.
Nothing more simple and reliable as a flathead. And they can make plenty of power, too.
A Tecumseh is also a good option, billet rod, valve springs, retainers, cam and slide carb will make a great runner. Lighted flywheel too if you're into that.
it's your bike obviously and it will be great no matter which way you choose!