A Loadstar 1603 is an older lighter duty bus chassis built from the early '60's to the late '70's.
The most common engine in the 1603 was the SV304 V-8 but there were quite a few built with the inline 6 that was less than 300 cubic inches.
That chassis was great for what it was. It was most comfortable going on route at speeds of less than 35 MPH. Some had 2-speed rear ends but most had single speed rear ends with the most common top speeds being 47 MPH and 57 MPH. Some of the heavier duty Loadstars had the SV345/392 or MV404/446 and they had gearing for better than 60 MPH.
I don't think any of the 1600's ever came with diesel engines unless it was the little Perkins. Most of the diesels were in the 1700 or 1800 series chassis. The ones that did have diesels had the 9.0L, the 6.9/7.3/T444, or the DT466.
Repair parts are still available for just about anything on a Loadstar. I have found most of the parts at my local NAPA, CARQUEST, or O'Reilly's. Brake parts can be a challenge but as a last resort you can try a Navistar truck or IC Bus dealer.
The line setting ticket should be available from www.binderbooks.com