The best money you can spend one your VMX hobby after fixing the bike up to start with is on good suspension and good tyres. They will have a much bigger affect on your enjoyment and performance than JT riding gear 
Sorry Ted couldn't help myself.
You can improve your suspension along the lines that Viper666 describes and the improvement is worthwhile. I just feel that instead of going along that path and then later upgrading the internals it is better to do the upgrade straight up. It tramsforms the suspension from 30yrs old suspenders that are well used even if freshened up into something that is as close to 'modern' performance as possible. It does cost a bit of money but most people head down that path eventually anyway.
PD valves in front forks with new linear springs to suite your weight.
Valve upgrade (gold valve type of gear can be fitted to YZ-D or so I've been told) for rear mono shock with new bump stop and spring
One of the Heaven Pre78 riders could not believe the transformation those upgrades made to his YZ400D and was looking forward to doing the same to his YZ125D
Yes you have to look at cost versus benifit but doing it right the first time saves money in the long run and increases the enjoyment of the ride regardless of what position you finish. Just my thoughts
You could always help the poor guy out Underbidder by " fixing his bike up to start with " with your Montessa race spec.
Faaark. Faaark , Shitbox Montessa, Faaark, push, push, pant, Faaark, pant, pant." Someone around here got a Jap bike they can lend me? " Faaaark, Faaark. " Get that camera outta my face." .Pant pant Faaark
