What year is this 450 John Deere Crawler ?

If it is a 450, the engines get bigger through the models. Early straight 450 diesel has a 202, later models go up to a 219 or 239. Late 450Bs have turbos.

If this is a 450C - and it has the HL-R hydraulic-shift transmission - coupled to a four-speed gear/range transmission - and you lose reverse in the HL-R when hot - walk away from it unless it's really cheap. You could easily spend $5000 getting it fixed. If the sellers tells you it's a "simple problem" - then tell him "good" - and to fix it before you buy it.

The HL-R trans is very rugged - but like anything else, it will wear out - and your's is over 40 years old. Parts for it have gotten incredibly expensive. So, often - when they start to slip - they go to auction. Especially early 450s since they aren't very powerful and not worth as much as the later versions. A late 450B 239 turbo will run circles around an early 450 with a non-turbo 202. Nice thing - for the sellers is - they usually work cold and don't start slipping or not-moving at all until hot. Same with the engine. That engine uses a pair of balancing shafts that often don't get fixed during normal "in-frame" engine rebuilds. When they get worn, and engine has great oil pressure cold - and gradually gets lower as it gets hot - sometimes down to zero at engine-idle. I had a chance to buy a 450 last month - with a winch and a 6-way dozer. The guy wanted $2800 - which normally would be a steal. But, he was also honest and told me that once it's hot - it won't move. I passed it up. Even doing all the work myself I could of spent well over $3000 on the trans - and who knows what else? That would make it a $5800 machine - if I assume all my labor is worth nothing. I'm not trying to make you feel bad - just telling you like it is. I hope you did't pay more than $2000 for it. If so - the only way out - is to send it to auction again - or - take it apart yourself and hope to get lucky.