Nice. Too bad the XS650 is almost unaffordable now. I just saw a Himalayan with a KTM RC390 transplant. That's a big power increase. Too bad it's a sucky engine with no grunt and sounds like a Briggs & Stratton.
At the risk of sounding self-serving, there are mods for the GT535(from 3 different sources, not just me) which can provide peak hp about the same as a KTM 390 Duke and still offer plenty of torquey grunt down low.
The reason that these smaller displacement lumps are weak down low is because they have inherently lower torque from their smaller displacement. They get their higher hp figures by revving that smaller tq to higher rpms, where the smaller tq gets multiplied by enough rpms to be strong in the upper revs.
Hp = tq x rpm/5252.
Simple as that. If you have low tq, just rev it to the moon to get your hp.
However, this is why people lament that kind of power curve as "no grunt".
The 535 has some grunt from tq, but it needs some revs for more hp. That's what we modders are giving it.
It doesn't need an engine swap.