My first recommendation would be to use your original front fork. I ended up trashing the new VBB fork I got from India. It was terrible, materials were poor and fitment to my bike was miserable. Purchase instead the Piaggio upper damper bracket on eBay (see photos) and then have it welded to your original fork. The precise location for this bracket I got from my brother's original 1965 VBB bike (photo). Next, purchase a new high quality swing arm. After trying two India swing arms, I ended up with a very good FA Italia swing arm. I also used high quality Japanese sealed bearings and bought a new axle to use with the new swing arm. I added new needle bearings for the fork pivot axle, new brake shoes, and a new Italian damper. I reused the original bearings and races for the steering column as they were in perfect condition. Their quality was so much better than the bearings that came from India. As for the spring, I used a Stoffis 155 mm spring primarily for their quality, durability, and ride. They are 20% stiffer and 33% stronger than the OEM Piaggio spring. Rob Hodge highly recommended this spring to me and that was all I needed to know.
The bike now rides extremely well with a nice firm ride that is not hard. The bike handles very well and is very stable. Compared to the original Allstate I had, the newly modified one is so much better in overall riding quality, handling, and stability on all types of roads. Would I do it again, the answer is YES. So if you have an old Allstate or know where you can get one, I would go for it knowing you can modify the front end and have a classic vintage Vespa. The uniqueness of the Allstate Vespa is also a plus in my opinion.
Below is a cost estimate of the many parts I used during this modification. Prices will vary but for the most part you can get an idea what such a modification would cost.
Upper Piaggio Bracket $16
(I had the upper bracket MIG welded to my original fork for $20)
Stoffis 155 mm spring $45
New modern front axle $35
(I initially used my original axle but it and the original main bearing just did not fit the new modern swing arm properly.)
New FA Italia swing arm $75 to $112 (this item seems difficult to get)
New front brake shoes $15
New sealed bearings for main axle (high quality Japanese sealed bearings were obtained from Amazon.com)
Circlip and oil seal (oil seal may not be needed if sealed bearings are used) $8
Suspension front pivot kit $25
Suspension spring axle kit $36
Front Italian damper and support hardware $48
The total estimated cost for new parts to convert the Allstate fork to a modern, state-of-the-art fork with a damper, is approximately $360. Shipping costs would be extra. This estimate dollar amount seems high when you compare what a complete VBB fork would cost on eBay from India. But based on my experience, in the long run, the parts I have outlined above will yield the best and safest front end.
Upper bracket for damper.
Precise fitment of upper bracket to original Allstate fork before welding.
Correct position of upper bracket based on original VBB fork.
Finished original Allstate fork with upper bracket.
New FA Italia swing arm.
New sealed main bearing in swing arm.
Completed Allstate front end with modified original fork, new swing arm, Stoffis spring, and damper.