Hope someone can help

I have just fitted a new engine in MY94 WRX, as the old one went pop. Got it all in and wired up and it started first time, bit lumpy as to be expected, however rather loud knocking noise coming from one sode of the heads. Phoned where i got it from, they told me it was the lifters and to just drive it, and iot will go. So have driven it for about 15 mins gentle not going over 3000/3500rpm.

However this is not the problem i now have, when i went to start it again the next day, i started it up and it fired first time as normal, and i let run and after about a minute and a half it made a noise and cut out, and wouldn't start agiain! After numerous checks around have now found the casue to be the timing belt has slipped/jumped about 3-4 teeth on only one of the pulley's from what i can see.

It is now booked in to have a new belt and timing done next week, but myt question is this,

what are the chances that this new recon engine will be dead now, and I'll need another one ?

note i did not touch the belt as the engine was supplied with a new belt and was run and tested before it was sent out to me

any help or comments would be appreciated