Stock tires (OK, tyres...) were 3.00S18 front and 3.50S18 rear.
Several makers have decent tires that will work, fronts are fairly easy to get in 3.50 or 90/90, larger than original but they look pretty decent. A 4.00 or 100/90 rear looks pretty big.
Metric tires don't exactly match inch sizes. A 90 width tire is 3.54 inches, 100 is 3.93.
The K81 is a nice tire and gives good results when used on both ends, but the smallest current size is 4.10, big on the rear rim and way too big for the front. Dunlop also makes the K82, it is very similar to the K81 but has a speed rating S (112 mph) while the K81 is H (130 mph). For a 400F the S rating would be adequate. Note that H rated tubeless tires derate to S with tubes usually - the K82 is a tube tire and rated S with tube. The K82 comes in all the smaller sizes you'd want. Although a block tread design, the K81/82 has a centre groove that causes squirrely handling on roads with "rain grooves" ground along the direction of travel. They are better for this than rib type fronts though.
K82 tires are available in the UK but doubtful in North America.
