only 4 grand?.. you could build a bike from used bits off ebay for about 1500 quid.. 4spd motor,exhaust alloy rims/tyres new. controls, tankmight be douable for a grand. for your grand plus you ll be wanting a new ignition, sprockets, chain, , brake shoes. that 1500 quid soon gets eaten up.
4/5k MIGHT get you a running v5'd donor bike a full drayton kit ignition wheels and with used wheels /forks.
all motorcycle trials is expensive.. pre 65 is potentially the most expensive and ironically sometimes the cheapest.