Hey there! There are a few threads about skid steers here and at
www.lawnsite.com If your thread here can't help you, read around and hopefully you'll find something that works.
First of all, building motocross tracks. If this involves hauling dirt around, spreading it, building it into large mounds, and moving ramps, a 763 might be a little small for you. For your information, by the way, the 763 has been replaced directly by thr S150. The S160 has 1600 lift and a 56 horsepower, turbo engine, but is the same size as the S150. The 160 might be a better fit for you -- if not a larger machine, period.
The Case 1840 has also been replaced; in this case, by the 40XT. The 40XT has a larger engine and is physically larger than was the 1840. It isn't significantly larger, but the added weight and power will help you quite a bit.
I would actually recommend going a little larger if you can afford the increase in cost. The Cat 236B is a very nice machine and should have all the power you will ever need. It's still a radial lift machine, which means you can dig quite a bit without worrying about the lift arm being weakened. It has an anti-stall feature which will allow you to dig without worrying about stalling your engine. Physically it is not too large, but should be large enjoy to handle your digging and lifting needs.
If you want a vertical lift machine, it will be difficult to beat the Cat 232B or Deere 317. The 232B is similar to the 236B but it has a vertical lift path and fewer horsepower. The 317 is still new; it hasn't really been commercially tested yet and probably won't be popular for another few months.
I'm not sure what you do to build a motocross track so can't really tell you what you need. I recommend considering a few aspects of skid steer design, however. First, the dealer itself. Is it interested in your business? How willing is it to help you and demo its equipment? Second, the equipment itself. Does it have a good reputation? Can it handle the tasks you will use it for? Third, the smaller aspects of the skid-steer. Servicability, operator comfort, mudability, handling. These can only be tested when you demo -- not by product literature. Make sure you test some out before deciding on one to buy.
Good luck. Let me know if you need any more information.
www.bobcat.com
www.cat.com
www.deere.com
www.casece.com (note: I am not sure if they sell SSLs in Australia)
www.newhollandconstruction.com (note: I am not sure if they sell SSLs in Australia)
www.thomasloaders.com (note: I am not sure if they sell SSLs in Australia)
www.gehl.com (note: I am not sure if they sell SSLs in Australia)
www.mustangmfg.com (note: I am not sure if they sell SSLs in Australia)
www.komatsuamerica.com (note: I am not sure if they sell SSLs in Australia)
www.jcbna.com (note: I am not sure if they sell SSLs in Australia)
www.volvoce.com (note: I am not sure if they sell SSLs in Australia)